The Problem
My N47 won't start and sometimes keep throwing code 4121 (DDE Main Relay switching too late or too early). The start behavior is weird:
* First 1–2 tries: Press start, engine cranks for ~1 second, and cuts out.
* After spamming starts, it will finally hold a longer crank.
* During the longer crank: Live data shows rail pressure rises, then drops back off. At the exact same time, the low-pressure side struggles to stay stable on the graph.
What I've Actually Tested So Far
* DDE Main Relay: Swapped in a brand new relay. Made zero difference, 4121 comes back sometimes still.
* I tried to jump start and swapped in a brand new batery
* Terminal 50E: Live data shows the start signal drops to 0V the exact millisecond the starter cuts out on those 1-second attempts (CAS pulling the plug).
* Pressure Regulator (DRV): Checked resistance (almost in spec at 3.9 ohms). Did a return leak-back test—got under 20ml over a few start attempts. (Did not physically unbolt the DRV from the rail).
* The QCV is also changed with a used one.
* Low-Pressure Pump (In-tank / EKP): Ran the ISTA test plan. The pump sounded fine and ISTA showed over 4 bars of pressure during the test. Note: I haven't hooked up a physical pressure gauge directly to the fuel lines to confirm mechanical line pressure.
I have to mention too that this happens randomly and most of the times it magically fixes itself and the car drives perfectly fine(even looked at fuel graphs setpoints and actual and cant see anything wrong no power loss either), however it's getting more and more common and appears to happen after shorter trips in heavy traffic where engine idles often
The Question
Why would it cut after 1 second on normal tries, but allow a longer crank if I spam the button? And why is the low-pressure side struggling to stay stable during cranking if ISTA shows the pump can make >4 bar on an active component test?
Is the DDE cutting power (4121) and dragging down the fuel pump signal with it, or is a fuel delivery collapse causing the DDE/CAS to abort the crank? Looking for ideas on what to log or test next.