We have two first gen Mazda 6 S 3.0L V6. Both cars drives fine but the AT + CEL Light will come on from time to time and goes into limp mode. Both cars also have the U0100 code.
I am not sure if this is related, but it seems that this happens very often when we park in an slight incline.
If you shift the transmission to park, pull up the handbrake and shut off the engine without letting off the foot brake, the car will move a bit. This will make the car jolt the next time you start the car and you shift it out of park. This often triggers the AT light. We have to shut off the car and restart it again.
If you shit the transmission to park, pull up the handbrake and let off the foot brake before shutting off the engine, everything goes smooth. We can drive both cars for a very long time without problems until our procedure of shutting the car off in inclines is “wrong”.