Peugeot 407 Ignition Problems

I had similar problems with my 407. From your description, it sounds like something in the car's electronics, specifically the ignition system. Rough running suggests a potential issue with fuel delivery after the start, possibly related to the immobilizer shutting things down unexpectedly. It is important to exclude defective relays, as the relay may cause the engine to shut off randomly. Did you check for error codes with an OBD2 scanner when the engine does not start, or when it shuts off? These errors might help you exclude certain electronic components.

Thanks for the info. I don't have an OBD2 scanner. If the car is running, the dashboard light is on as normal, but when it happens, turning the key does absolutely nothing - no click, no whirr, nothing. Do you think it's still worth getting it scanned, or is it likely something else?

Given that you're experiencing a complete lack of response when turning the key at times, it could be the ignition switch itself failing mechanically, or an electrical fault like corroded contacts. The key not being recognised might also have something to do with it. While scanning might not reveal the exact fault when it’s completely dead, it would still be a good idea to bring it to a professional to have them have a look, especially with a potential immobilizer problem, so you don't get stranded somewhere.

Just took it to a workshop, and you were spot on with the ignition switch. Apparently, it was faulty. New ignition switch and labour cost me 52€ and the car is now running fine. Thanks for the advice!