Seat Leon Diesel Preheating Issues

I had something similar happen with my Leon, though I can't recall the exact details. The glow plug light stayed on, and the engine light popped up too. Have you checked the connections to the vehicle heater module? Those can get pretty grimy.

Yes, I meant the preheating control unit, sorry for not being clearer. If the light is staying on and you are getting smoke, I would seriously consider getting it checked out by a proper mechanic, especially since you suspect that the heating module might be faulty. It is probably the control unit.

Interesting. When you say 'vehicle heater module,' are you referring to the preheating control unit itself, or something else entirely? I’m wondering if it could be linked to the car electronics.

It was a defective preheating control unit, just like you suspected. The mechanic confirmed the cable and plug connections were corroded, causing the error. They replaced the control unit, and the total cost was 95€. Thanks a lot for your help!