G6 transmission control unit failure

Sounds like a real headache! When you say it won't go into park, does that mean the shift lever moves to the P position, but the engine still doesn't shut off and the car can roll? Or does it physically not move into the P position? Does the error memory display the same code every time? Also, can you describe when the gearbox slips?

Yeah, the lever moves to P, but the engine keeps running and it rolls! The error code is always the same, something about a transmission control unit. The slipping happens mostly when it's cold, feels like it's struggling to find the right gear.

Okay, the fact that it rolls in park and the engine keeps running definitely points to a defective transmission control unit affecting the parking pawl engagement. And the slipping when cold further supports that. It's like the brain that tells the transmission what to do is scrambled. Based on those symptoms and that mileage, it's highly probable. Unfortunately, with that specific error code cropping up, you're most likely looking at a costly repair. You’re going to need someone to diagnose that control unit; I would take it to a workshop.

Thanks a lot for the help! Took it to the shop and you were spot on. The control unit was indeed defective. Cost me 1000€, but at least it's fixed!