C5 Torque Converter Lock-Up Clutch Issue

Indeed, the symptoms you describe strongly suggest an issue with the torque converter lock-up clutch. Is the engine RPM surging erratically at cruising speeds, particularly when maintaining a constant throttle position? A defective lock-up clutch can certainly manifest in the ways you've described, triggering those error codes and affecting shift quality. Have you already checked the transmission fluid level and condition?

Thanks for the quick reply. Regarding the engine RPM, yes, there is some surge. Fluid level seems OK, but it's due for a change soon. Does this confirm the converter issue, or are there other potential causes that exhibit the same symptoms?

While a fluid change might offer marginal improvement, the presence of grinding noises and slippage, coupled with the engine surge and error codes, strongly implicates a failing torque converter lock-up clutch. Could other issues cause similar symptoms? Conceivably, but the combination of these factors points decisively to the converter. I would recommend getting a professional diagnosis and repair quote. Ignoring it will cause major damage to the gearbox.

Thanks for your expertise! I appreciate the detailed confirmation. I took it to a workshop. You were right, it was the torque converter. Repair cost was 2120€. Everything is working fine now.