Almera preheating lamp stays on

Have you inspected the glow plugs themselves? A faulty glow plug can cause the preheating lamp to stay on and trigger the check engine light. The smoke could be unburnt fuel due to inadequate preheating. Also, can you confirm the engine is running smoothly, or are you experiencing any misfires or stalling?

Thanks for the tip. I haven't checked the glow plugs directly, I will do that. As for the engine, it runs mostly fine, but when it's cold, it definitely struggles a bit and feels like a slight engine misfire. Could a bad glow plug really cause all these problems?

Yes, a faulty glow plug or related wiring issue is a plausible cause for all of those symptoms. Given the age and mileage, it's certainly worth investigating thoroughly before assuming the control unit itself is faulty. However, considering the misfire and smoke, a workshop visit might be beneficial for a proper diagnosis and repair. They'll have the tools to test the glow plugs and control unit properly.

Thanks a lot for the help! I took it to the workshop, and you were spot on. It was the cable or plug connection of the glow control unit that was defective. They fixed it for 95€ and everything is working fine now. No more smoke!