Tourneo diesel preheating lamp issue

Sounds like a pain, like having a leaky tap you can't quite turn off. When you say the preheating lamp stays on, is the engine running rough, especially when it's cold? Any check engine light on the dash? If the glow plugs are gone, it can mess with the whole diesel combustion process.

Yes, it's a diesel car and the engine is a bit rough when starting. And yes, the check engine light is indeed on. Do you think it's definitely the glow plugs, then?

Given the symptoms and the fact you've got the engine light on, it does sound highly probable that the glow plugs are the issue. Like having a dodgy lightbulb that keeps tripping the breaker. I'd say get it checked at a workshop; better safe than sorry. It's a diesel engine, so glow plugs are pretty important, and if they are faulty it can cause problems down the road.

Thanks for the confirmation and the advice! I took it to the workshop, and indeed, the glow plugs were defective. The repair cost me 440€. Appreciate the help!