Fiesta preheating lamp stays on

Sounds familiar. I had similar symptoms on my old Opel Astra, though it was a petrol engine. The check engine light and persistent preheating indication are dead giveaways for an issue in that area. Have you checked the wiring harness and connections to the heating module for corrosion or damage?

Thanks for the insight. I did a visual inspection of the wiring and connections, but didn't find anything obvious. Did your Opel show any specific error codes related to the preheating system?

Yes, it did throw some codes related to the glow plug circuit. After some deeper digging and testing it became clear that the electronics were at fault. I would suggest going to the workshop for further investigation and repair.

Thanks! Took it to the workshop, and you were spot on. Turned out to be a defective preheating control unit, just as I suspected. Cable and plug needed replacement. Total damage was 95€ Appreciate the help!