E Klasse Diesel Engine Preheat Issue

I had a similar problem with my E Klasse. The engine preheat system was acting up. Have you checked the glow plugs themselves? A faulty glow plug can sometimes trigger those symptoms and throw codes related to the heating module.

Thanks for the suggestion. I haven't checked the glow plugs directly. Is there a straightforward way to test them without removing them from the engine? Or is it best to just replace them all?

Testing them in place is difficult. You'd need a multimeter and know the acceptable resistance range. Given the symptoms and mileage, it's probably best to take it to a workshop to have the car electronics diagnosed correctly. A defective engine preheat control unit will cause smoke, the check engine light to come on and the preheating lamp to not switch off.

Appreciate the advice. Took it to a workshop, and you were right – it was related to the preheating control unit. Turns out it was a corroded connector. Repair cost 95€ including labor. Thanks for pointing me in the right direction vehicle heater is working again.