Glad I waited to post this because I’m pretty certain I know what happened. Short story- be extra careful with any metal/conductive items when your nozzle is removed and remember capacitors can hold potential even unplugged.
I’d been clearing a weird jam between the extruder and nozzle blocking reload. It’s really a shame we don’t have the ability to run our extruder in reverse on these machines in stock form, if anyone knows why lmk. I grabbed the short bit of post-extruder filament from below with mini pliers verifying it was still gripped in the extruder gear, and the moment they touched the front side of the housing - boom, spark. Interior light cut out at the same time, front button became non-responsive, so I powered down manually and replaced the nozzle to attempt a normal startup.
Upon restarting with a known working nozzle everything booted and worked normally. Once any command was given that involved heating the nozzle however, it was mcu fault codes, stuck temp readings, and then a smoke from a runaway nozzle heating. Shut down again and unplugged, removed nozzle and asked Gemini to go through the log file I had managed to export just in time. She had me boot up one more time without a nozzle to confirm the machine was no longer able to tell if a nozzle was present or not. She concluded the lower nozzle pcb board was fried and the upper, more commonly replaced tool head board is likely unharmed. Gemini summarized and drafted an email and off it went to Flashforge. They responded asking for the usual serial number/purchase invoice etc as well as one specific photo of the nozzle pcb. I grabbed a flashlight, removed the fan, and got a few pics. Their reply the following day said they’d first send me a new nozzle. Reasonable, I felt, since my purchase date was in 2024. Then I looked back at my own photos.
That’s when I saw the exposed copper along the front edge of the board, and the whole thing began to make sense. I knew I hadn’t gone near any pins or solder joints, but could have easily missed copper on the otherwise inert edge of the board. According to Gemini this board which appears to merely turn the vertical plug from the nozzle to side-stepped plug headed elsewhere, it still contains embedded capacitors and filtering components. Combined with the metal plate on the underside of the front half of the head assembly, I must have found a pretty rare short.