Terrano preheating lamp issue

I had similar symptoms in my Terrano. The preheating control unit manages the glow plugs, which heat the cylinders for starting. If the lamp stays on, it suggests the unit isn't correctly regulating the glow plugs or detecting their temperature. Have you checked the voltage at the glow plugs themselves and also the resistance?

Voltage and resistance? I'm not really familiar with measuring those. Where exactly should I be checking on the glow plugs?

The glow plugs are screwed into the cylinder head. Disconnect the electrical connector on each glow plug, then use a multimeter to measure the voltage with ignition on. Afterwards, measure the resistance of each glow plug; a high resistance means the plug is likely faulty. That being said, considering the check engine light and smoking, it would be best to visit a workshop. It could save you a lot of headaches.

Thanks for the advice! I took it to a workshop and it turned out you were right about the control unit. They found a faulty connection. 95€ later, it's fixed.