Peugeot 307 preheating lamp and check engine light

Sounds familiar! I had a similar issue with my 307. Have you checked the glow control unit? Those symptoms could point to a bad connection. Could be as simple as a loose cable or a corroded plug.

Thanks for the quick response! I've checked the control unit, but how can I diagnose a bad connection exactly? Do I need a multimeter or something?

A multimeter would definitely help to check for continuity and voltage, but honestly, those connectors on the 307 can be a pain. If you're not comfortable digging too deep, and you're sure it's the glow control unit, maybe just take it to a reliable workshop for proper diagnostics. It might save you a headache.

You were right. I took it to a workshop, and they found a defective cable in the glow control unit circuit. Repair cost 95€. Thanks for pointing me in the right direction!