Boxer preheating lamp and check engine light

I had similar issues with my Boxer, though I don't remember the year. Persistent preheating lamp, engine light, and smoke... sounds familiar. Did you check the glow plugs? Are they all working? Also, have you inspected the wiring harness for damage or corrosion?

The glow plugs are okay, and the wiring harness looks fine. But could a defective cable or plug connection cause these symptoms, like you experienced?

Yes, a faulty connection can cause those issues, even if it's not obvious. But given the symptoms and your suspicion, the preheating control unit itself may be the problem. If you don't have diagnostic tools, a workshop visit might be best.

The preheating control unit was defective. A replacement and labor cost 95€. The symptoms are gone, and the van is running well again.