V70 preheating lamp check engine light

The symptoms you describe certainly point towards a potential issue with the glow control unit. Think of it like this: the glow plugs are like a tiny internal heater for your engine, especially important for cold starts with a diesel. When the control unit malfunctions, it's like the thermostat in your house getting stuck. Some questions to consider, has the car's performance been impacted and are cold starts problematic?

Thanks for your reply! The car's performance hasn't been noticeably impacted, and cold starts are only a little problematic as they take a bit longer. Does this information help to narrow down the problem?

Indeed, it does. Based on your description and the fact that cold starts are only slightly problematic, it is highly likely that you are dealing with a defective glow control unit, perhaps stemming from a cable or plug connection issue as you initially suspected. Given your description of the problem, and for a definitive diagnosis, I recommend consulting a qualified workshop. They will be able to conduct thorough testing and confirm the root cause.

Thank you very much for the detailed explanations and suggestions. I went to a workshop, and you were right. The glow control unit was indeed defective due to a corroded connector. Repair costs were 95€. Everything is working fine again!