RCZ Automatic Transmission Shifting Issue

Undervoltage can mess with modern cars and their electronics. Your symptoms sound like transmission issues from bad solenoid operation or corrupted data to the TCM. Before blaming the transmission, check the charging system, battery, alternator, and wiring. Also, check ground connections; bad grounds cause undervoltage.

Thanks. I checked the battery and alternator, and they seem okay. Could voltage fluctuations, instead of steady low voltage, cause these intermittent transmission faults? How would I diagnose these voltage drops?

Fluctuations can cause issues with sensitive electronics. Get a diagnostic scan with a tool that can monitor live data from the TCM. Watch the transmission input and output shaft speeds, solenoid signals, and TCM voltage. If it keeps happening, get it to a workshop that knows automatic transmissions.

Thanks for the advice. I went to a workshop, and they said the transmission oil was worn out, which they blamed on the voltage fluctuations, even though it's supposed to be 'lifetime fill'. A transmission oil change (240€) fixed it. It shifts normally now, and the error messages are gone.