Peugeot 107 preheating lamp stays on

That sounds familiar. I had a similar problem with my old Opel Astra, a petrol version though. The preheating light stayed on, and it turned out to be a faulty glow plug. Although our engines are different, the symptoms you describe are very similar. Have you checked the glow plugs themselves?

When you say 'checked the glow plugs themselves,' what exactly do you mean? Is there a way to test them without specialized equipment? I'm not particularly experienced with car repairs, but I'm willing to try if it's something relatively straightforward.

What I did was using a multimeter to check the resistance of each glow plug. A healthy glow plug should have a low resistance. If one shows a significantly higher resistance, it's likely defective. I'm not sure if it's the best approach but it worked for me. However, considering your lack of experience, maybe it's best to have a workshop take a look. It could save you some headaches and prevent further damage. I ended up paying 350€ back then. I've now heard that this would cost me 440€ for my Peugeot 107.

I appreciate your detailed explanation and advice. I think you're right; it's probably best to take it to a professional. I'll book an appointment with a workshop and have them diagnose and repair the glow plugs. Thank you again for your help!