Peugeot 107 Transmission Indicator and Check Engine Light

Radiator oil pressure? That's not usually a thing. Are you sure they didn't say radiator coolant pressure? There's a pressure sensor in the cooling system. A defective cable or plug connection to that sensor could cause the issues you describe, as the ECU might be getting false readings and throwing things into limp mode. Does the cooling fan seem to be running all the time or not at all? That could also indicate a problem with the coolant pressure gauge and would align better with your symptoms.

Coolant pressure! Yes, that's probably it. Sorry, I'm not a mechanic, and this is all new to me. So, if the cooling fan is acting weird (I'll need to check that!), that could be directly linked to the gearbox problems? I'm just trying to understand the connection. Why would the ECU care about coolant pressure when it comes to shifting gears?

The ECU monitors a lot of things, and unusual readings can trigger safety protocols like emergency mode for the transmission. A faulty coolant pressure sensor could send incorrect data to the ECU, which then affects other systems. It's not a direct link, but the ECU interprets the sensor data and reacts accordingly. To be sure, I would go to a professional workshop. Replacing a faulty cable/plug connection to the pressure sensor on your own may be possible, but you should definitely leave it to the experts.

Thanks so much for the explanation! That makes a lot more sense now. I checked the cooling fan, and it was running constantly, even when the engine was cold. Took it to the workshop, and you were spot on - a defective plug connector on the coolant pressure sensor was the culprit! 95€ later, everything's back to normal. Really appreciate the help!