Megane preheating lamp issue

The symptoms you describe are consistent with a malfunctioning preheating control unit. The preheating system is crucial for cold starts in diesel engines, and a faulty unit can indeed cause incomplete combustion, leading to smoke and illumination of the check engine light. Have you checked the glow plugs themselves, or just focused on the control unit and its connections?

Good point! I've been so focused on the preheating control unit itself (and the potential wiring issues) that I haven't checked the glow plugs. Is there a simple way to test them without pulling everything apart? I'm assuming a multimeter would be involved, but what readings should I be looking for?

Yes, a multimeter is indeed the tool for the job. You should be looking for a low resistance reading across each glow plug. Significantly high resistance, or an open circuit, indicates a faulty glow plug. However, given your symptoms, especially the persistent preheating lamp, a faulty control unit remains a strong possibility. It might be prudent to consult a workshop for a diagnostic check, as further electrical faults may be present.

Thanks for the advice! I had a local mechanic check it out, and it turned out to be a defective cable connection to the preheating control unit, just as I suspected. Replacing the cable and plug cost me 95€ and solved all the issues. Appreciate the help!