Sorry that I've been quiet. I went to Cape Town to visit my brother and got a second bout of COVID.
Thanks for the offer of a TCU, KondorRed, we'll see how it goes.
Back to the story...
I took out the original TCU which was throwing occasional Check Auto Gearbox errors. I split it open. NOT an easy job and I got a good deal more energetic with levers than I intended. Fortunately, while the software may be soft it doesn't seem to be brittle...
I couldn't see anything damaged inside. I concentrated on the MOSFETs (none of which looked cooked) and the large capacitor. I was interested to see that the Chinese reconditioned unit still had the original capacitor as there was original adhesive on it as well as new. The back of that unit has (I think) a coil on it and therefore I assume that the whole thing smooths power surges, but I am not an electrician. The new capacitor was soldered in whilst the original just had grip fittings. The old capacitor still held a good charge: take heed!
I couldn't think of a way to test the steppers. There is a very high resistance across the terminals, so there must be a driver or something in there.
The computing board, if it is, was extremely reluctant to separate from the back board, so I left it in there. But what IS replaced in the Chinese reconditioned units? I see no evidence that those boards have been repaired and I'm sure that replacement cost would be prohibitive. So at the moment it seems that neither the boards nor the capacitor are replaced. Are the stepper drivers replaced? I don't know.
So putting all these thoughts together I soldered the new capacitor onto the back board fitting and put it together with the new steppers.
To my surprise the car started and runs fine without errors. I made a 5 km loop with the gearbox working perfectly. We'll have to see in the longer term.
When it fails again I will try swapping the two clutch servos. I have spares of those. I would have thought that a failed clutch operation COULD cause any of the errors that I have seen.
Sorry that there wasn't a more concrete result.