Lexus CT200h transmission slipping

That sounds like a potential issue with your automatic transmission. Grinding noises and slipping aren't good signs. Have you noticed any changes in fuel economy or engine RPM at highway speeds? Does it happen in all gears, or just some? When was the last time you had the transmission fluid checked or changed?

Fuel economy seems normal, but the RPMs do fluctuate a bit at highway speeds. It seems worse in the lower gears. I haven't had the transmission fluid checked recently. Would low fluid cause these symptoms? Also, do you think the torque converter is the root cause, or could it be something else?

Low fluid could contribute to some of those symptoms, but grinding noises usually point to something more serious inside the transmission itself. Based on the error code and the symptoms you're describing, a defective torque converter is definitely a strong possibility. The lock-up clutch inside the converter is a common failure point. At this point, it really needs to be diagnosed properly at a workshop to confirm that's the problem and estimate the full extent of the transmission repair needed.

Appreciate the advice. I'll take it to a reputable workshop and have them diagnose the torque converter. It's looking like it will be about 2120€ to fix. Thanks again!