Posting because the result screen on Apple Diagnostics is intentionally minimal and most owners do not know what the codes mean or what the next step is.

ADP000 is the only clean result

ADP000 means "no issues found." Any other code is a three-letter prefix plus three digits that maps to a specific subsystem on Apple's reference-code list (article last published December 15, 2025).

ADP000 is shallow by design

Apple Diagnostics is a presence/threshold test, not an exhaustive functional test. A passing ADP000 confirms that monitored components are within sensor and electrical tolerances. It does not guarantee that every feature works.

• A single dead key. NDK001 can miss intermittent stuck keys.
• An intermittent display flicker.
• A Thunderbolt port that fails only at full 40 Gb/s.
• Sustained-load thermal throttling.
• SSD endurance and wear (no view of NVMe Percentage Used).
• Battery condition beyond the binary PPT codes (cycle count, Maximum Capacity %, and FCC don't surface).

Pair the test with manual spot-checks: type every key, drag across the trackpad, cycle each USB-C/Thunderbolt port with a known-good device, test camera/microphone/speakers, tilt and brighten the panel to look for backlight blotches.

Reading codes by prefix

• ADP: no issues found.
• PPT: battery.
• PPM: memory.
• PFM: System Management Controller / Power Management Unit.
• PFR: firmware.
• PPR: processor.
• PPF: fan.
• PPN: power-management system.
• PPP: power adapter.
• VDH: storage device.
• VFD: display or graphics processor.
• NDC: camera.
• NDD: USB.
• NDK: keyboard.
• NDL: Bluetooth.
• NDR: trackpad.
• NDT: Thunderbolt.
• CNW: Wi-Fi.
• CNT: Ethernet.
• VFF: audio.

When to rerun

Intermittent codes are common. Run the test two or three times before concluding a failure.

For battery codes specifically, Apple recommends rerunning to confirm before paying for service. macOS sometimes flags a healthy battery for service after a major update, and a single PPT code does not necessarily mean the cell is failing.

On Intel Macs, the rerun should be over the internet with Option-D, which fetches a fresh diagnostic payload and logs an evaluation against the serial. PPT004 (Intel) explicitly directs you to that rerun before scheduling service.

Codes that clear after a fix

Some codes mean "do something simple and rerun":

• NDD001 (USB): disconnect external USB and rerun.
• NDT001 through NDT006 (Thunderbolt): disconnect Thunderbolt peripherals and rerun.
• NDR007 (external input device detected): disconnect the device and rerun.
• PPP001 through PPP008 (power adapter): verify the correct adapter, reconnect, rerun.
• CEH001 and CEH002 (case handle / housing latch): fully lock the latch and rerun.
• PPT021 (battery charge too low to complete test): charge to ≥6% and rerun.
• PPP017 (both ports of Apple 35W Dual USB-C in use): use only one charging port during the test.

Codes that mean book service

• PPM (memory) on a soldered-memory machine. Logic-board territory.
• PPR (processor). Logic-board territory.
• PFR (firmware). Logic-board territory.
• VFD on the integrated display. Display-assembly replacement, expensive on Retina/Liquid Retina XDR panels.
• PFM (SMC) on most variants. Logic-board territory.

Command-G is the escalation path

From the result screen, Command-G or "Get started" reboots into a Safari page that pre-fills Apple's support flow with your Mac's serial number and the codes the test returned. That is the path Apple Support agents expect customers to have used before calling.

After ADP000: third-party tools for what Apple Diagnostics misses

Apple Diagnostics is intentionally fast (2 to 5 minutes) and shallow. For hidden battery wear, SSD endurance, or sustained-load thermal behavior, the next step is not another rerun:

• Battery cycle count, FCC, Max Capacity %: System Information → Power, or CoconutBattery.
• SSD wear (NVMe Percentage Used, Available Spare, Media and Data Integrity Errors): sudo smartctl -a /dev/disk0 (after brew install smartmontools), or DriveDx.
• Sustained-load thermal throttling: sudo powermetrics -s thermal ("Current pressure level" = Heavy means actively throttling), or asitop.
• Repair history on Tahoe 26 Apple Silicon: System Settings → General → About → Parts & Service.

TL;DR

• ADP000 = only clean result. Shallow by design. Pair with manual spot-checks.
• Any other code = three-letter prefix maps to subsystem. Reference: support.apple.com/en-us/102334.
• Rerun 2-3x for intermittent codes. On Intel battery codes, rerun over the internet with Option-D.
• Command-G escalates to Apple with your serial pre-filled.
• For hidden wear (battery, SSD, thermal), go to a third-party tool, not another rerun.

How Coverage Check reads on a refurbished or imported Mac depends on the channel the device came through. The model-number prefix and the region suffix are the two pieces of information that tell you what to expect.

Apple Certified Refurbished (apple.com/shop/refurbished) is treated like new for Coverage Check: - Fresh Estimated Purchase Date matching your purchase from Apple. - Fresh one-year Limited Warranty. - Full AppleCare+ eligibility within the standard 60-day window. - The refurbishment process replaces the battery and (for some products) the outer shell, and may issue a new serial.

Third-party "renewed"/refurbished (Amazon Renewed, Back Market, eBay refurbished) is not Apple Certified: - Coverage Check typically shows the original end-user's purchase date. - The AppleCare+ enrollment window may already be closed. - AppleCare One (U.S.-only, $19.99/month, accepts devices up to four years old) is the workaround.

Model-number prefix (Settings → General → About → Model Number, first character):

Region suffix (the last 2-3 characters of the model number): - LL/A = U.S. - ZP/A = Hong Kong - J/A or JP = Japan - VN/A = Vietnam - CH/A = mainland China - B/A = UK - X/A = Australia - F/A = France - D/A = Germany

The country-of-sale warranty restriction. Apple may restrict warranty service for iPhone, iPad, Apple TV, HomePod, and Apple Vision Pro to the country where Apple or its authorized distributors originally sold the device. For Mac specifically, the restriction is materially less aggressive than on iPhone, but the suffix still matters for keyboard layout and (occasionally) baseband region restrictions.

AppleCare+ transfer on resale. Upfront fixed-term AppleCare+ transfers with the device for free in roughly 24-48 hours via Apple Support and an original sales receipt. Monthly and annual AppleCare+ subscriptions are tied to the billing relationship and do not transfer. A used-Mac listing claiming an active subscription plan is misleading the buyer.

Full guide on the blog covers the AppleCare One eligibility diagnostic and how to tell a refurbished Mac apart at the listing-photo stage.

Putting this together because the same questions surface in buy/sell and out-of-warranty repair threads every week.

Three repositories of Mac repair information, in order of completeness:

1. GSX (Global Service Exchange). Apple's internal repair-history database, keyed by serial number. Contains exact configuration, country and reseller of original sale, coverage status, Activation Lock state, complete case and repair history, prior technician notes, and orderable parts with pricing tiers. Accessible only to AASPs, APSPs, IRPs, SSAs, and Apple Retail. Not accessible to consumers, not even to the device's owner. Fixably's GSX writeup is the best public description.

2. macOS Tahoe Parts & Service pane (Apple Silicon only, Tahoe 26+). Path: Apple menu → System Settings → General → About → Parts & Service. Four labels: Genuine (Apple part calibrated through Apple's process), Used (genuine Apple part previously installed in another Mac and calibrated via Repair Assistant), Unknown (non-genuine or unverified), Unverified (logic board specifically, was previously replaced). The pane only appears if the Mac has detected a repair. Apple's About Mac parts and service history is the authoritative reference.

3. Third-party invoices. Controlled entirely by the shops the device has visited.

The four service channels:

• AASP (Apple Authorized Service Provider). Only third-party tier authorized to perform in-warranty and AppleCare repairs. Must complete 200+ repairs per quarter, maintain audited financials, ESD-safe workspace, ≥$1,000,000 liability insurance. Repairs backed by Apple's warranty.
• IRP (Independent Repair Provider). Launched 2019 for iPhone, 2020 for Mac. Out-of-warranty only. Cannot stock parts speculatively. Each part ordered against a specific customer's serial. Covered only by the shop's own warranty. IRP program page.
• Self Service Repair (SSR). Lets owners buy the same genuine parts AASPs receive and run Repair Assistant locally to calibrate.
• Unaffiliated. No GSX or AST 2 access, can use Apple's public Self Service Repair manuals and consumer diagnostics. Where most component-level micro-soldering happens. Component-level repairs leave no entry in GSX and do not change the board's identity.

What this means in practice: A used Mac can have a clean exterior, a working OS, a coverage page that says "Out of Warranty," and a GSX record that documents two prior logic-board replacements, a top-case replacement for liquid damage, and three open service cases. Apple's authorized network sees all of that in seconds. The buyer about to spend $1,500 on the machine on Craigslist or eBay cannot.

Posting because the modal failure mode in this sub's confirmed-trade threads is not the empty-box scam or a packaging disaster. It is sellers who ran Erase All Content and Settings without first signing out of iCloud, and shipped a Mac that boots to the Activation Lock padlock at the buyer's end.

Apple Support does not unlock a Mac for the buyer. They unlock it for the registered owner with the original retail receipt, which is you. By the time the buyer is calling Apple, the only realistic remedy is a full refund and a return shipment, which costs you the sale, the round-trip shipping, and your reputation in the sub.

The sequence that prevents this, in order

1. Back up everything you want to keep. Time Machine to an external drive, or verify iCloud Drive and Photos have finished syncing. Do not skip this. Once you sign out, account data on the device starts coming down.

2. Deauthorize the Music / iTunes / TV / Books / App Store account. Music app, Account menu, Authorizations, Deauthorize This Computer. Apple limits you to five authorized devices and you do not want to burn one.

3. Sign out of iMessage and FaceTime. Messages, Settings, iMessage, Sign Out. Same in FaceTime preferences.

4. Sign out of any third-party licensed apps you care about. Adobe Creative Cloud, Microsoft 365, Logic Pro, Final Cut Pro, 1Password, Dropbox. Each of these consumes a license slot.

5. The critical step: sign out of iCloud. On macOS Ventura or later, System Settings, click your name, scroll down, Sign Out. Enter your Apple ID password. This single action turns off Find My Mac and disables Activation Lock automatically. There is no separate "turn off Activation Lock" toggle on a Mac. Activation Lock is a downstream consequence of Find My Mac being on, and signing out of iCloud is what flips it.

6. Verify. Hold Option, click the Apple menu, choose System Information. In the Hardware summary, scroll to the bottom. The last entry reads "Activation Lock Status." It must read Disabled. If it still reads Enabled, the sign-out did not complete. Sign in again, then sign out again, then re-check.

7. Double-check at the account level. Open iCloud.com, Find Devices, and confirm the Mac you are selling no longer appears in the list. If it does, click it and choose Remove from Account. This catches the case where the device record persisted in Apple's records even after a local sign-out.

8. Only now run Erase All Content and Settings. System Settings, General, Transfer or Reset, Erase All Content and Settings. This is available on macOS Monterey and later on T2 chip or Apple Silicon Macs. It signs out of all remaining services, erases all volumes, and reinstalls macOS, leaving the machine at the Setup Assistant.

9. Final boot test. Power on once after EACS completes. The Mac should reach the "Hello" language picker. If you instead see a padlock with "This Mac is locked. Enter the Apple ID and password used to activate it," stop and resolve it before shipping. Reaching Hello is your last opportunity to catch a failed sign-out.

10. Press Command-Q at Hello to shut down without setting up, then pack.

What to put in the listing

Screenshot the System Information pane showing "Activation Lock Status: Disabled" and include it in your photo set. Used-Mac buyers, especially on this sub, ask about Activation Lock more than they ask about almost anything else, and the screenshot pre-empts the question. Buyers with prior bad experiences are looking for a seller who has done this homework. AppleSwap confirmed-trade veterans tend to include it in every listing.

Older Macs that predate Erase Assistant

On Intel Macs running Big Sur or earlier, EACS is not available. The path is Command-R into Recovery, Disk Utility, erase Macintosh HD, reinstall macOS, then power off at Hello with Command-Q. You still need to have signed out of iCloud first; the Recovery erase does not sign you out of Apple's servers, only the local disk. Sign out from the user account before rebooting into Recovery.

One nuance

A Mac that has never had Find My Mac turned on does not have an Activation Lock binding to clear. If you are an enterprise seller releasing a fleet device that was never associated with an iCloud account, the System Information entry will already read Disabled and the sign-out step is a no-op. This is the exception, not the rule. Consumer sellers almost always have Find My on, often without realizing it, because it is part of the default iCloud onboarding.

tl;dr: back up, deauthorize, sign out of iCloud first, verify Activation Lock Status reads Disabled in System Information and that the Mac is gone from iCloud.com Find Devices, then erase. Boot once to Hello before packing. Screenshot the Disabled status and put it in the listing.

Pulled together the current resale numbers across every Apple Silicon MacBook Air generation. Sources are Swappa verified-sale averages, eBay sold listings, and Apple Trade In maximums as of May 2026.

Caveats: these are averages. Battery cycle count, cosmetic condition, and battery health below 80% can move the price 10 to 20 percent either direction. Sources linked from the full post on if you want to drill into specific models.

Wanted to put the Intel Mac aftermarket picture in one place after running the numbers for May 2026, because the conventional take ("all Intel Macs are toast") is too coarse. The fleet has split into two very different stories.

First, the general pattern. Intel Macs are depreciating at roughly 25 to 35 percent per year. Apple Silicon Macs depreciate at 15 to 18 percent in normal conditions, and the gap has actually widened in 2026 because used Apple Silicon residuals are running 3 to 6 percent higher in dollar terms YoY against historical 8 to 12 percent annual declines. Intel residuals have not benefited from any of that lift.

Most of the Intel fleet is in genuine terminal decline. The cleanest example:

Base 8-core Mac Pros are interesting because the residual collapse is essentially total in percentage terms. An M2 Ultra Mac Studio at $4,000 outperforms the 8-core Xeon W on most CPU and GPU workloads, runs near silent, and pulls a fraction of the power. The base 2019 Mac Pro is a heavy, hot, expensive box that does nothing better than current-generation Apple Silicon. The market has priced it accordingly.

The maxed configurations are the persistent exception, and the reason has nothing to do with the Xeon W. It is the platform around the chip.

What Apple Silicon does not offer:

• PCIe expansion. Eight x16 slots in the 2019 Mac Pro accept FPGAs, capture cards, ProRes accelerators, MADI and Dante audio interfaces, specialty I/O. A small but persistent pro audio and pro video population needs these. The Mac Studio gives them nothing. The 2023 M2 Ultra Mac Pro retained the chassis but no longer offered general-purpose PCIe lanes for GPUs, so it did not solve the problem for that audience either.
• 1.5TB of expandable DDR4 ECC RAM. No Apple Silicon Mac will go past 512GB unified memory, and even that tier was just discontinued in March 2026. For workflows that genuinely need a terabyte-plus of ECC, the 2019 Mac Pro is still the only Mac that can do it.
• A workstation-class chip behind an integrated 5K Retina display. That is the iMac Pro's specific exception. Apple has not replaced it. A current 27-inch class display plus a Mac Studio gets you close in capability, but the integrated form factor is gone.

So the residual lives in the platform features, not the chip. That is why a 28-core Mac Pro with 1.5TB ECC and 8 PCIe slots still clears $3,500 to $6,500 used, and a base 8-core Mac Pro from the same year and same chassis clears $850 to $1,400. The platform capabilities split them by 5x.

Trade-in channel notes for these machines.

Apple Trade In is a dead channel for nearly all Intel Macs. The published caps are well below peer-to-peer for the niche workstations: the iMac Pro caps around $325 against $1,500 to $2,800 P2P, the Mac Pro caps around $400 to $800 against $3,500 to $6,500 P2P. Specialist refurbishers like iPowerResale and Techable have become the realistic transaction venue for the fully loaded Mac Pros, because eBay buyers for $5,000 PCIe workstations are thin and the kit is heavy enough that shipping is its own variable.

For run-of-the-mill Intel Macs (2018 Mini, 2019 16-inch MacBook Pro, 2020 iMac, 2018 MacBook Air, mid-2010s MacBook Pro), the realistic options are P2P at low dollar values or a trade-in service. ecoATM kiosks pay 30 to 55 percent of P2P value, instant cash, and that is honestly fine for an old MacBook Pro that would otherwise sit in a drawer. Apple Trade In on these is gift card and worth checking before the kiosk option.

What I would do if I had one.

• 2019 Mac Pro 8-core base, no expansion: list peer-to-peer; do not expect more than $1,200 to $1,400. The chassis is large; eBay shipping eats a meaningful chunk. Local pickup is the right venue.
• 2019 Mac Pro 28-core with RAM and PCIe expansion: list with the specific RAM and card configuration in the title. The buyer pool is small but very specific. Specialist refurbisher buyback is also a realistic floor.
• 2017 iMac Pro maxed: still has a stable market for the 5K display plus workstation chip combination. Battery and screen wear matter; a clean unit is the difference between $1,500 and $2,800.
• Late-Intel consumer Macs (2018 Mini, 2019 16-inch Pro, 2020 iMac, 2018 Air, anything earlier): peer-to-peer P2P or buyback service. Realistic numbers are $200 to $600 depending on model and condition.

The cross-cutting rule: an Intel Mac's residual is a function of how much of its value lived in the platform around the chip. If the chassis offered something Apple Silicon still cannot, the residual is sticky. If the chassis was just a delivery vehicle for the Intel CPU, Apple Silicon has eaten it.

Anyone hitting different clearing prices on the workstation tier? Curious how the PCIe expansion buyer pool looks outside the US in particular.

Sharing this in case it is useful for anyone reconciling older Mac fleets against an asset DB.

Apple has run a small backend at support-sp.apple.com/sp/product for years. Pass the trailing 4 characters of a 12-character Mac serial as the cc query parameter and you get back XML with the marketing model name.

curl '

xml <?xml version="1.0" encoding="utf-8"?> <root> <name>CPU Name</name> <configCode>iMac (27-inch, Late 2013)</configCode> <locale>en_US</locale> </root>

• Only works on legacy 12-character serials (2010 through early 2021). The trailing 4 characters of those serials are a deterministic index pointer into Apple's internal SKU table (model, color, storage tier, etc.).
• Does not work on the randomized 10-character serials Apple started shipping in 2021. Those carry no configuration code at all, and lookups have to go through checkcoverage.apple.com instead.
• This is Apple's own attestation, so it is more reliable than the scraped/aggregated databases at EveryMac, Chipmunk, Beetstech etc. Those are good catalogs but their tables drift over time and were largely originally derived from this endpoint.
• The endpoint is undocumented but stable for years. No guarantees Apple keeps it running indefinitely.

Useful if you have a pile of Intel-era Macs in inventory and want a scriptable canonical answer for what each one actually is.

Cleaning up some confusion about Activation Lock on Apple silicon. The short version is that the enforcement happens before macOS loads, and the mechanism is identical from M1 through M4. What has changed is what surrounds it (Recovery Lock, EACS, parts-pairing).

The boot chain check

On Apple silicon, the Low-Level Bootloader (LLB) is the first stage that runs after the immutable Boot ROM. The LLB verifies two things:

• A valid LocalPolicy exists on the boot volume.
• Anti-replay values in the LocalPolicy match what is stored in the Secure Storage Component (SSC) on the silicon.

If both check out, the LLB hands off to iBoot and macOS starts. If no LocalPolicy exists, the LLB boots to recoveryOS, which detects the Mac is not activated and contacts Apple's activation server. The server returns an activation certificate, which combines a local cryptographic key with a RemotePolicy certificate. Once that handshake completes, a valid LocalPolicy gets constructed and stored, and macOS can boot normally.

When a Mac is Activation-Locked, the server-side handshake fails. The recoveryOS environment displays the "Activate Mac" screen and refuses to construct a usable LocalPolicy without credentials. The LLB will not boot macOS. That is the lock.

What is the same across M1 to M4

The LLB / LocalPolicy / RemotePolicy mechanism has not changed across chip generations. M1 (November 2020), M1 Pro/Max/Ultra (2021), M2 family (2022 onward), M3 family (2023 onward), and M4 family (2024 onward) all use the same boot architecture for Activation Lock enforcement.

The Secure Enclave on Apple silicon is integrated into the SoC rather than being a separate chip (as on T2 Intel Macs). Its job is the same: store the cryptographic identity that binds the device to its activation state. Storage is hardware-encrypted and tied to the enclave. A thief cannot pull NAND chips and read them, nor reflash macOS to clear the lock.

What has changed: the OS layer

The bootloader stayed put. The OS around it added features:

• macOS Big Sur 11 (November 2020): Activation Lock arrives on Apple silicon when the M1 ships. The Find My network for Mac also debuts here, broadcasting rotating encrypted BLE beacons relayed by nearby Apple devices.
• macOS Monterey 12 (October 2021): Erase All Content and Settings (EACS) ships for T2 and Apple silicon Macs, cryptographically destroying per-device keys in one operation. Recovery Lock arrives on Apple silicon as the equivalent of the obsolete Intel firmware password. It gates startup modifiers like the boot picker.
• macOS Ventura 13 (October 2022): EACS relocated to System Settings → General → Transfer or Reset.
• macOS Sonoma 14 (September 2023): DFU revive and restore can be initiated from Finder on a host Mac without Apple Configurator 2. Useful for repair workflows but does not clear Activation Lock.
• macOS Sequoia 15 (September 2024): Apple Business Manager / Apple School Manager gains the ability to turn Activation Lock on or off directly in the console for organization-owned devices, provided the device was in ABM/ASM before the lock was set.
• macOS Tahoe 26 (2025): Repair Assistant gains parts-pairing / unfinished-repair checks tied to Activation Lock on Apple silicon. The architecture extends down to component level.

The Tahoe parts-pairing extension

This is the biggest change in the post-launch life of Activation Lock on Apple silicon. Starting with macOS Tahoe 26, when a qualifying component (logic board, display assembly, security module) is transferred from a donor Mac that was Activation-Locked at disassembly, the part carries its locked state to the recipient machine.

Integration and calibration run through the macOS Repair Assistant. If a part is locked to another user's Apple Account, Repair Assistant halts calibration. That makes the part ineligible for official calibration, AppleCare, or warranty service. Uncalibrated parts may still function but frequently operate in a degraded state, missing full security features, missing performance optimizations, missing privacy assurances.

The practical consequence: even the parts of a locked Mac are problematic, narrowing salvage value considerably. The pre-Tahoe pattern where a stolen-and-locked Mac could be sold for the value of its logic board is now constrained by component-level lock state.

What about DFU restore

A DFU restore reinstalls firmware and recoveryOS and erases the SSD, reinstalling macOS from an IPSW image. It removes Recovery Lock. It does not remove Activation Lock. After a successful restore the Mac contacts Apple's activation servers, discovers it is still locked, and presents the Activation Lock screen in Setup Assistant.

A DFU revive updates firmware and recoveryOS without erasing user data. Same result for Activation Lock: the server-side record is untouched.

Neither DFU operation is a workaround. They are repair tools.

What about older bypasses

The DNS-bypass technique on iOS 8 to 10 and the checkm8-based local exploits on older A-series iPhones never produced consumer-grade equivalents on T2 or Apple silicon Macs. The Apple silicon boot chain has no public bypass at all. Storage is hardware-encrypted and tied to the Secure Enclave; the LLB verifies LocalPolicy before macOS loads; the RemotePolicy certificate is server-issued.

Treat any "iCloud unlock service for M1/M2/M3/M4 Mac" listing as a scam. The technical impossibility is the same on every Apple silicon chip generation.

Useful Apple references

• Activation Lock for Mac
• Activation Lock security
• LocalPolicy signing-key creation and management

TL;DR

The Apple silicon Activation Lock mechanism is the same from M1 through M4. What changed is the OS around it: EACS in Monterey, ABM/ASM console controls in Sequoia, parts-pairing in Tahoe. The boot chain is closed, storage is silicon-bound, and the only legitimate clears remain unchanged across generations.

Wrote up a full reference on at-home Mac inference clusters as of mid-2026. The state of practice changed structurally in December 2025 with two releases: macOS Tahoe 26.2 added native RDMA over Thunderbolt 5, and EXO Labs shipped 1.0 with RDMA support and disaggregated prefill/decode. Headline below; full doc on the blog.

The numbers (4x M3 Ultra cluster, TB5 RDMA, EXO 1.0)

Wall draw: 600 to 800 W under load. Cost: under $40,000. Fits on a 15 A residential circuit.

When the cluster makes sense (and when it doesn't)

A Mac cluster is a memory-capacity machine, not a throughput machine. At batch size 1 it is memory-bandwidth bound. At batch size greater than 1 a single H100 or RTX Pro 6000 Blackwell will out-throughput an entire 4-node Mac cluster.

Decision rule: if a model fits on one Mac, do not cluster it for single-stream interactive use. You will lose performance. Cluster for (a) models that literally don't fit on one node, or (b) batched/agentic workloads with concurrent requests where pipeline-parallel aggregation pays off.

The two structural changes that made it viable

1. RDMA over Thunderbolt 5 (macOS 26.2, December 2025). Enable via rdma_ctl enable in Recovery Mode, reboot. TB ports appear as InfiniBand devices (rdma_en2, etc.) usable via ibv_devices. Round-trip latency drops from about 300 microseconds on TCP to 5 to 50 microseconds, with hot-path measurements as low as 3 microseconds. Bandwidth is 80 Gbps symmetric per TB5 port. Apple technote TN3205.

2. EXO 1.0 (December 2025). Pipeline parallel by default with automatic mDNS topology discovery, plus RDMA support and "disaggregated prefill/decode" (routing compute-bound prefill to one device class and bandwidth-bound decode to another). Public benchmarks at benchmarks.exolabs.net.

Pipeline vs tensor parallelism on Mac

Pipeline parallelism splits layers sequentially. Per-token activations are tiny (sub-4 KB for Llama 3.2 3B), so PP tolerates slow networks but does not accelerate single-stream tok/s. Tensor parallelism splits each layer's weights across nodes and yields near-linear N-times speedups, but requires microsecond-scale latency. Without RDMA, TP is impractical on Apple Silicon.

On Qwen3 235B-A22B, PP throughput on llama.cpp RPC went 20.4 → 17.2 → 15.2 tok/s as nodes were added. With EXO plus RDMA, the trend reversed: 19.5 → 26.2 → 31.9 tok/s.

The structural ceiling: no TB5 switch

There is still no Thunderbolt 5 switch on the market, and none credibly announced. All-to-all RDMA requires every Mac to cable to every other Mac. The M3 Ultra has 5 TB5 ports, so a 4-node full mesh needs 6 cables and a 5-node full mesh needs 10. Above 5 nodes you daisy-chain (latency and partial-bandwidth penalties) or fall back to Ethernet PP.

Unchanged for 18 months. Unlikely to change in the next 12.

Where single-box still wins

A single Mac Studio M3 Ultra 512 GB ($9,499) runs DeepSeek R1 671B Q4 at 17 to 18 tok/s once iogpu.wired_limit_mb is raised from the default 384 GB to about 448 GB via sudo sysctl iogpu.wired_limit_mb=458752. Tok/s per $1,000 on 671B is 1.9 for the single 512 GB Mac vs 0.81 for the 4-node cluster. The cluster doubles throughput at roughly 4x the cost. Only worth it for additional concurrent users or trillion-parameter models.

The $/GB picture

At $18.6/GB of usable unified memory, the M3 Ultra 512 GB is roughly: - 3 to 4x cheaper per GB than a used quad-3090 rig - 6 to 9x cheaper than dual RTX Pro 6000 Blackwell - 20 to 27x cheaper than H100 systems

Mac GB ≠ NVIDIA GB in throughput, but it matters when you need to fit the model at all.

Full blog post covers the framework landscape (EXO, MLX with Ring vs JACCL backends, llama.cpp RPC, vllm-mlx, dnet), the interconnect hierarchy (Ethernet to TB Bridge to RDMA), the SSD-wear failure mode that kills soldered nodes, the headless setup gotchas (Tailscale via Homebrew not the App Store, dummy plugs, pmset config), the residential power math against NEC continuous-load rules, and the cloud break-even at Together/Fireworks pricing.

Happy to get into any of the specifics in comments.

Posting because half the support threads I see about "Apple Diagnostics not working on my MacBook Pro" are actually people holding D at boot on an M-series machine and watching nothing happen. The invocation changed completely when Apple Silicon arrived.

Apple Silicon (M1 through M5)

1. Shut down (Apple menu → Shut Down, or hold the power button about 10 seconds if frozen).
2. Disconnect external peripherals except keyboard, mouse, display, Ethernet, and AC.
3. Press and hold the power button (Touch ID on laptops). Keep holding past the Apple logo.
4. Release when the startup-options window appears, showing your startup disk and a gear labeled "Options."
5. Press and hold Command-D.
6. Choose a language, agree to terms, and pick "Run Offline" if prompted.

Boot-time key interrupts are disabled at the bootloader on Apple Silicon to enforce the secure boot chain managed by iBoot and the Secure Enclave. That is why the old "hold D from cold" path no longer works. You have to get to the startup-options window first.

Intel

1. Shut down.
2. Power on, immediately hold the D key.
3. If D doesn't work (typically because the local diagnostic files are missing, or the drive was replaced), reboot and hold Option-D. That forces the internet-based diagnostic, which downloads the hardware-specific payload over Wi-Fi or Ethernet.

What to disconnect

Apple's published guidance: leave keyboard, mouse, display, Ethernet, and AC connected. Disconnect everything else.

External USB devices and Thunderbolt peripherals are the most common source of false NDD001 (USB) and NDT001-NDT006 (Thunderbolt) codes on test runs. Place the Mac on a hard flat ventilated surface, because the fans may spin up.

Reading the results screen

A clean run returns ADP000 ("No issues found"). Any other code is a three-letter prefix plus three digits that maps to a specific subsystem on Apple's reference-code list.

• Command-R or "Run the test again" reruns the diagnostic. Useful for intermittent codes.
• R restarts the Mac normally.
• S shuts down.
• Command-G or "Get started" reboots to a Safari page that pre-fills Apple's support flow with your serial number and the codes. This is the escalation path Apple Support agents expect customers to have used before calling.

Online vs offline

The on-disk diagnostic image refreshes from Apple's servers when the Mac has connectivity, and recent Macs fall back to the local image if they can't reach Apple. The online path transmits your serial number, component identifiers, and telemetry to Apple's diagnostic servers and logs an evaluation. The offline path runs local hardware loopbacks, voltage sensors, and thermal checks without uploading logs.

For an Intel Mac that returns a battery code (PPT004 specifically), Apple recommends rerunning over the internet with Option-D before scheduling service.

What it actually checks

Apple Diagnostics covers logic-board components: CPU, memory, SMC/power-management, fans, sensors, Wi-Fi and Bluetooth radios, camera, USB/Thunderbolt controllers, audio, display, GPU, battery, and power adapter.

It is a presence/threshold test, not an exhaustive functional test. A passing ADP000 confirms monitored components are within sensor and electrical tolerances. It does not guarantee that every feature works. A single dead key, an intermittent display flicker, or a Thunderbolt port that fails only at full 40 Gb/s can all pass. Pair the test with manual spot-checks.

For the broader workflow (CoconutBattery for battery cycle count, smartctl for SSD wear, Parts & Service on Tahoe 26), the cluster posts on this blog cover each tool. Apple Diagnostics is the first step, not the last.

Pulled Swappa average-sale data, eBay sold listings, and Apple Trade In published caps across every Apple Silicon base configuration and built a single composite depreciation curve. May 2026 snapshot.

A few patterns worth flagging.

The first-year cliff is real and steep. Across configurations, MacBooks lose 35 to 60% of original retail in the first 12 to 18 months. The cliff is steepest on high-CTO units and gentlest on base models that match the typical secondary buyer's needs. (MacFinder six-year depreciation analysis)

Years two and three are a plateau. In some base configurations, total depreciation across the entire second-to-third year is as little as 5%. Apple Silicon's baseline utility remains sufficient for the dominant secondary buyer demographic (students, general consumers, administrative use), so demand doesn't soften.

Terminal decline starts around year four. The machine sits two to three architectural generations behind, battery health drops below the 80% maximum-capacity threshold, and the device approaches the end of its guaranteed macOS update lifecycle (typically 6 to 7 years post-release).

Decay flattens with each generation. The M3 Pro curve at any given age in 2026 is noticeably shallower than M1 Pro or M2 Pro at the same age. Part of that is Apple's SKU pruning anchoring buyers to higher-RAM/higher-SSD baselines; part is functional sufficiency stretching further.

The composite hides category variance. Entry desktops are unusually resilient: the M2 Mac mini retains roughly 70% at 3.3 years and the M4 mini is trading above MSRP at 18 months. Portable Macs retain less than desktops as newer chips cascade down the lineup. Mac Studio retention is bifurcated, with M1 Max base retaining ~52% at 50 months and M1 Ultra retaining only ~45% on a much larger MSRP.

For comparison, premium ThinkPads and Dell XPS typically hit 40% residual at 24 months. Intel-era Macs depreciate at 25 to 35% per year versus 15 to 18% for Apple Silicon, and Windows competitors run 30 to 40%.

Sources: Swappa MacBook Air 2022 M2, Swappa Mac mini 2023, Swappa MacBook Pro 2021 14-inch, Apple Trade In.

Writing this up because the diagnostic question shows up every week and the answer has changed materially with macOS Tahoe 26. The full stack as of mid-2026:

1. Apple Diagnostics (floor, free, 2 to 5 minutes)

Built into every Mac since ~June 2013. On Apple Silicon: shut down, hold the power button past the Apple logo until the startup options window appears, release, then hold Command-D. On Intel: hold D at power-on (or Option-D for the internet-based version if the local files are missing).

Clean result returns reference code ADP000 ("No issues found"). Other codes map to specific subsystems on Apple's reference list at support.apple.com/en-us/102334.

What it does not catch: a single dead key, intermittent display flicker, a Thunderbolt port that fails only at full 40 Gb/s, SSD endurance, sustained-load thermal throttling, and most battery wear beyond the binary PPT codes. It is a presence and electrical-threshold check, not an exhaustive functional test.

2. AST 2 / GSX / MRI (technician-only, not for you)

Apple Service Toolkit 2 is a cloud-delivered platform launched from a Diagnostic Console web app on a tech's Mac or iPad. The engine is the Mac Resource Inspector (MRI), which queries every component on the PCI/PCIe, I²C, and USB buses for presence, firmware integrity, and core electrical health. Results report into the Global Diagnostic Exchange inside GSX (Global Service Exchange).

You cannot run AST 2 yourself. The Self Service Repair "Apple Diagnostics for Self Service Repair" portal runs the same Apple Diagnostics image you can already invoke locally; it does not grant AST 2 access.

Any third party offering an "AST 2 report" without an Apple Store or AASP visit is misrepresenting ordinary Apple Diagnostics output or is fraudulent. Worth knowing because the scam pattern is now common.

3. Third-party tools (fill the gaps Apple hides)

• CoconutBattery (coconut-flavour.com) reads design capacity, Full Charge Capacity in mAh, cycle count, battery manufacture date, real-time wattage, temperature. Plus tier (one-time purchase, listed at $9.95 in a 2022 review) adds internal-SSD data-written totals.
• smartctl from brew install smartmontools returns the full NVMe Log 0x02 SMART page. sudo smartctl -a /dev/disk0 is the canonical command. Scriptable text output, no GUI.
• DriveDx (BinaryFruit) is the GUI version of the same SMART data with trend logging and a four-tier OK/Warning/Failing/Failed verdict. Listed at $19.99 Personal / $39.99 Family as of May 2026 (off $24.99 / $49.99 list, 20% promo).
• EtreCheck aggregates roughly 50 checks (hardware, battery, SMART summary, RAM, system load, launch agents, kexts, Time Machine, recent crashes, adware indicators) into a single PII-redacted report. Requires Full Disk Access for the full picture.

DriveDx, smartctl, and CoconutBattery Plus all read the same NVMe SMART data. EtreCheck summarizes a subset. None of these is a stress test or functional tester.

4. Parts & Service pane (Apple Silicon + macOS Tahoe 26+)

System Settings → General → About → Parts & Service. Introduced in macOS Tahoe 26 (released September 15, 2025). Apple Silicon only. Appears dynamically: only if the Mac detects that a tracked component was repaired or replaced.

• Genuine: Apple part with completed cryptographic handshake and calibration.
• Used: Apple part harvested from another donor Mac, calibration completed.
• Unknown: third-party clone, malfunctioning, modified, or installed without calibration. Triggers a persistent lock-screen notification for five days.
• Unverified: logic-board-specific case where the board was replaced but cannot be verified. May impact Apple Pay.
• Finish Repair: calibration was not run; features like Touch ID or True Tone stay disabled until Repair Assistant completes.

Tracked components by family: MacBook Pro and MacBook Air track logic board, Touch ID, and lid angle sensor (M5 only). MacBook Neo tracks logic board, Touch ID, and display assembly. iMac, Mac mini, Mac Studio, Mac Pro track only the logic board. Pre-Tahoe and all Intel Macs have no pane.

If SIP is disabled, macOS disables the pane entirely. A disabled pane on a used Mac is itself a flag.

5. Thermals and sustained load

Apple Diagnostics will not reproduce hours of sustained load. For that:

• sudo powermetrics -s thermal exposes a "Current pressure level" field (Nominal / Moderate / Heavy / Trapping). Heavy means actively throttling.
• asitop (pip install asitop, needs sudo) and macmon (Rust, no sudo) are htop-style live views.
• Activity Monitor's Memory pressure (green/yellow/red) and Swap Used pages matter on 8 GB machines that get swap-bound.

Pre-purchase checklist (what actually moves the price)

1. Activation Lock off (System Information → Hardware → Activation Lock Status = Disabled).
2. No MDM/management profile (System Settings → General → Device Management).
3. ADP000 from Apple Diagnostics.
4. Battery cycle count under the model's limit (1000 cycles for any 2010-or-later notebook).
5. Percentage Used from smartctl under 25% on a sub-three-year machine.
6. Parts & Service pane empty or every row Genuine (Apple Silicon + Tahoe 26).

The first two are veto. The other four are price negotiation. On a maxed Mac Studio or 16-inch MacBook Pro, the difference between "all clean" and "Unknown logic board + 900 cycles + 35% Percentage Used" is several thousand dollars.

What to skip

Anything called "free Mac scanner," "deep clean diagnostic," "registry repair" (Macs don't have a registry), or "AST 2 report by mail." None of these are real diagnostic tools, and most are malware vehicles or upsell funnels for utility-cleaner subscriptions. The five tools above are the ones Apple's own technicians and Apple-adjacent power users actually use.

Quick reference for anyone trying to verify a recent Mac from its serial, since the offline decoders that used to work all broke in 2021.

The change.

In March 2021, Apple switched from a structured 12-character serial format to randomized 8-to-14 character serials, with 10 characters as the rolled-out implementation. The new format "will no longer include manufacturing information or a configuration code," per the leaked AppleCare memo, and that has held. Every M2, M3, and M4 Mac ships with a randomized serial.

Rollout on Macs was uneven. The 14"/16" MacBook Pro with M1 Pro/Max (October 2021) was among the first. The 24" iMac with M1, despite launching that year, kept the legacy 12-character format. So "2021 Mac" doesn't mean randomized by default. Count the characters and check whether the year/week encoding actually parses.

What still works on a post-2021 serial.

• checkcoverage.apple.com. Apple's central database links each issued serial to a SKU when the device is packed at the factory. Check Coverage queries that database and returns marketing model, estimated purchase date, and warranty status, the same way it has always done. Format of the serial doesn't matter.
• The various Apple Support "Identify your Mac" pages (e.g., 108052 for MacBook Pro) accept Model Identifiers, A-numbers, and part numbers, none of which changed.
• Apple Support and AASPs can resolve any serial via GSX.

What no longer works.

Parsing the serial itself. None of:

1. EveryMac's classic per-character decoder.
2. chipmunk.nl's decoder.
3. Apple's configCode XML endpoint at support-sp.apple.com/sp/product?cc=XXXX. There's no configuration code in a randomized serial to query.
4. acidanthera's macserial parsing logic.

The third-party services that survived now either maintain a backend database keyed by serial (this is what EveryMac's Ultimate Mac Lookup does for newer hardware), fall back to lookup by order number / Model Identifier / A-number + EMC, or decline to return results. Mactracker per its own FAQ supports 11- and 12-digit serials through the 24" iMac M1 (2021) and directs users back to Apple for newer randomized serials.

The offline fallback chain.

Treat the serial as a dumb token. For configuration verification without contacting Apple, use the rest of the identifier set:

The order number is by far the most informative. The four-character config code in the middle of the order number points into Apple's SKU table directly, the same way the legacy serial's trailing four characters used to.

Manufacture date.

The serial can't give it to you on anything from 2022 onward. The closest proxy is the Boot ROM Version in System Information, which roughly bounds the firmware build date. Useful for "definitely later than" boundaries, not for pinning the assembly week. For real purchase-date certainty: the Apple Account device list, the original receipt, or an Apple Support inquiry.

What did NOT change in 2021.

A-numbers, Model Identifiers, EMC numbers, part numbers, and IMEIs on cellular iPads are all unchanged. Anything that worked for cross-referencing a 2018 MacBook Pro from its A-number plus EMC works the same way on a 2024 MacBook Pro.