AlexFW

Hello folks, I own a 2005 545i with 134k km on the clock. The car has generally behaved well until recently, though earlier this year I had some issues with corroded ignition coil connectors.
However, last month the very sporadic SMG errors that had been occurring for a while have become more frequent, with this display when they occur:



Loosely translated: gearbox in limp home mode, when you turn off the car it will not start again' (and indeed, if you try to start the car right away, it won't crank)


As an additional detail, about 3-4 years ago the gearbox was replaced with one from a sister car. Unfortunately I have no details on the miles or the age of the unit that was swapped in, though it did look reasonably good in pictures.

Battery has been checked and seems ok. Visual inspection of engine grounds did not reveal any glaring issues.

Further diagnosis has been very difficult because.. nothing is showing up on the diag pulls. The error with the gearbox, however, repeats itself randomly at first startup. The absolutely confusing part is that even WHEN the gearbox is in 'limp home' mode, there are no errors in the diag *and* the gearbox will allow you to use 2nd, which it should not. Shut the car off, wait ten minutes, restart... no problem. The only clue so far is that IF the car starts off without an issue, the issue doesn't show up afterwards. If it shows up at startup, it will stay until the car is turned off.

Current thought is 'test all the sensors electrically to see if one of them is having issues', but it's kind of a shot in the dark. I suggested adding the wiring loom and connectors to the sensors themselves.

Any help, any thought?

Thank you!