CMAX preheating issue

Whoa, sounds familiar! I've got a CMAX too, and I had a similar problem with the preheating. The glow plug light stayed on, and the engine was running rough. Did you check the glow plugs themselves? Sometimes it's not the control unit but one of those little buggers that's gone bad.

I haven't checked the glow plugs individually yet. Is that complicated? Any special tools needed? Can I check the wiring harness to the preheat controller without a multimeter?

Look, checking the plugs isn't rocket science, but it can be a pain to get to them. Honestly, with the check engine light on and the smoke, I'd just bite the bullet and take it to a decent shop. It could be a dodgy connection like you thought, or something else entirely. Best to get it diagnosed properly before you start throwing parts at it. Mine turned out to be a bad sensor, and it cost me a pretty penny to sort out after messing with it myself.

I took it to a workshop as suggested. They confirmed a defective preheating control unit, specifically a corroded connector. The repair was 95€ including parts and labor. Thanks again for the insights.