[SGspirit]

This happens to our A604. The symptoms are that it always happens during a drive. It's never in limp mode when we start out.

It happens at irregular intervals ranging from a few days to a couple of months. Usually it happens when coming up to speed after being at a stop, but very rarely it will happen while driving along continuously. Usually there is a little "lurch" that happens just before it malfunctions.

There are never any trouble codes. Stopping and shutting the car off and on again clears the problem. I can do this while driving by shifting into Neutral as long as the situation is very safe and I'm not going too fast.

I've cleaned the main connector on the front of the transmission, with no effect on the problem. The mechanic who rebuilt the transmission has been unable to figure it out.

 

[SGspirit]

Limp mode from the moment the car starts has a good chance of being an electrical/sensor problem.
Limp mode that happens after a lurch is likely because of a low pressure situation. Low pressure causes hard shifting. The revised TCM was more tolerant of low pressure and adapted accordingly resulting in less hard stuffing, bump stops and limp modes.

Ours shifts very softly. The mechanic who rebuilt it and has checked for this problem is an automatic transmission specialist who is regarded as one of the best in our area. I expect he'd have the right scanner etc. It's just that the malfunction cannot be reproduced on demand, and it's infrequent enough that I can live with it.

Just have to be careful about being in the left lane in heavy traffic when starting up from a stop light on a road that goes to highway speed, because I've gotten stranded in limp mode in the fast lane.