C2 Automatic Transmission Fault

Grinding noises and slipping gears sound bad. I had something similar on an old car. The check engine light was flashing. It turned out to be a sensor, not the transmission. Did you check the transmission fluid level? Low fluid can cause weird issues. Also, did you scan the error memory for the exact codes? That might give you a better clue than guessing at the torque converter. Thinking it’s just the lock-up clutch from those symptoms seems like blaming a leaky faucet for a flood in the basement.

I checked the transmission fluid level, and it's fine. I also scanned the error memory, and the fault code indicates a definitive issue with the torque converter lock-up clutch. Does that error code change your assessment, or does it still seem like it could be something simpler?

Okay, a specific torque converter lock-up clutch error code does change things. If the computer is directly reporting an issue with that component, it's much more likely that's the problem, not some random sensor or low fluid. While I'm no mechanic, that points to a serious internal transmission problem. Honestly, at this point, I think you should take it to a trusted mechanic for a proper diagnosis. Continuing to drive it with these symptoms could cause even more damage.

Following the error code confirmation and your recommendation, I consulted a mechanic. Their assessment agreed with my initial thought: the automatic transmission converter was indeed defective, specifically the lock-up clutch. The repair cost 2120€.