Saxo diesel preheating lamp issue

Sounds like a possible issue with the glow plug system, yes. The symptoms you describe are pretty typical. Before condemning the control unit, have you checked the wiring and plug connections to it? A bad connection there can cause all sorts of weirdness. Is there any diagnostic test available?

Thanks for the quick reply! I haven't checked the wiring yet, but that's a good point. Where exactly should I be looking for these connections? And is it safe to drive it like this in the meantime?

The control unit is usually near the engine block, follow the glow plug wires back, you'll find it. Inspect for corrosion or loose connections at the plug. Regarding driving, it's probably okay for short trips, but continuous driving with a faulty preheating system can strain the engine and the catalytic converter. Get it checked soon. Better go to the workshop to do a diagnostic test.

Thanks a lot for the advice! I checked the connections like you suggested, and there was a corroded connector. Cleaned it up, but no luck. Took it to the workshop, and they confirmed it was a faulty cable/plug. 95€ later and it's all sorted! Thanks again for pointing me in the right direction.