EDIT: Turns out it was a failed DME. I've had it repaired and all issues are resolved.

EDIT 2: On 12/7/2023 the issue returned, this time while driving, which has never happened before. I'm waiting to hear back from the company I sent the DME to for repairs.

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Original post:

I have a 2000 540i 6MT with 90k miles exhibiting a no-start condition, combined with the following:

1. Engine Failsafe Prog warning in the cluster
2. Temp gauge immediately pegs red on cold key insertion
3. Throttle motor is silent
4. Starter motor engages and cranks

I've already replaced the dual temp sender in the thermostat housing, and checked the wiring for shorts and discontinuities. Wiring is fine, and the condition occurs across the original sender and a new Delphi unit.

Here's where things get weird. All the relevant fuses (14, 15, 17, 22, 31, EF1-5) are intact and have solid continuity, but if I remove all 5 engine bay fuses and reinstall them, the engine will start and run normally. However once it's turned off, the condition returns, and will only start again if all 5 fuses are removed and reinserted. Presumably, the "disconnect the battery for at least 16 minutes" trick will have the same effect, but I haven't tested it since getting as far as the engine bay fuses. Resetting Fuse 14 does not have any effect.

Incidentally, the battery is good and maintains >12v during cranking. The issue occurs with the car connected to a battery tender as well as unplugged, so I do not suspect a battery-related issue at this time.

INPA does not reveal any codes or errant live data. Presumably, all the "hard-resetting the power on the DME" is contributing to this, though I'm trying not to speak into existence the obvious implication.

It's about to rain, so I've buttoned everything back up to keep water out, but in the meantime, any ideas?