Peugeot 408 Preheating and Smoke Issue

Sounds familiar! I had similar preheating issues with my old Opel Astra a while back. Preheating lamp stayed on, engine light came on, though no smoke in my case. Have you checked the voltage at the preheating plugs? Sometimes a bad relay can cause similar symptoms, like a short circuit creating a feedback loop.

Interesting. I haven't checked the voltage yet, focused on the obvious connection points. Could a bad relay really cause the smoke too? Seems unlikely, but possible. Where is the relay located normally?

Well, the smoke could be coincidence, or maybe unburnt fuel due to the preheating system messing with the fuel mixture – like when your fridge freezer is not working correctly, nothing is as it should be. On the Astra, the relay was near the battery, but Peugeot probably tucked it away somewhere different. Honestly, given the mileage and symptoms, and unless you are sure of your diagnosis, I'd suggest a workshop visit. A diagnostic tool will pinpoint the issue quickly.

Thanks for the insights. I followed your advice and got it checked at a workshop. Turns out it was indeed a defective preheating control unit and dodgy connector, just as I suspected. 95€ all in, problem solved!