C2 preheating lamp stays on

Sounds familiar! I had similar symptoms with my old Opel Astra, a real headache. Turned out to be some weird electronics issue. Have you checked the cable connections and plugs on the heating module itself? Could be a bad connection.

Yeah, I gave them a look, but maybe not thoroughly enough. Where exactly should I be focusing my attention on the control unit? Any specific cables known to cause trouble?

Honestly, I'm no expert, but my mechanic said it was a corroded plug on the main preheater relay. The thing is, it's hard to diagnose without proper tools. Considering the symptoms, I'd suggest taking it to a workshop; a faulty preheating control unit could mess up the engine pretty badly, or could be a bad connection as you say.

Thanks! Appreciate the insight. I took it to a local garage; you were spot on! They checked all the connections, found a corroded plug, cleaned it up, and replaced a faulty wire. Cost me 95€, but everything is working like new again!