Seat Inca Diesel Preheating Lamp Issue

The symptoms you describe do indeed point towards a glow control unit issue, or at least something affecting it. The fact that the preheating lamp stays on, coupled with the check engine light and smoke, suggests the engine isn't getting the correct preheating for a clean start and efficient combustion, especially when accelerating. Have you checked the fuse for the glow plug circuit? Also, I remember I had a similar issue once with my old Passat, turned out to be a corroded connector on one of the heating element. Do you have a multimeter to check the continuity of the wires going to the glow plugs and the control unit?

Thanks for your quick response! I did check the fuse, and it seems to be fine. I don't have a multimeter, but I did inspect the visible parts of the wiring. What exactly should I be looking for when checking the continuity of the wires? Is there a specific reading I should be getting?

Since the fuse is fine, and you've visually inspected the wiring, it does sound more and more like a defective cable or a bad connection to the glow control unit. Without a multimeter, it's difficult to pinpoint the exact problem. Given the symptoms and your suspicion, I'd strongly recommend taking it to a workshop for a proper diagnostic test. They'll be able to check the control unit itself and the wiring harness with specialized equipment. It's better to get it sorted properly, especially with the cold start issue potentially damaging the engine over time. The heating element needs to work properly.

The problem is solved. It was indeed a defective cable to the glow control unit, just like you suspected. The diagnostic test at the shop confirmed it. They replaced the cable and the glow control unit itself, and the preheating lamp now switches off as it should. Check engine light is gone, and no more smoke when accelerating. Cost me 95€, which is acceptable I think. Thanks again for your help!