Peugeot 607 running rich

Whoa, sounds rough! I had something similar happen with my 607 (not sure about the year, it's an older one). Check Engine light was on, and the fuel consumption was insane. Did you check the oxygen sensor? Could be a faulty signal messing with the fuel mixture. Also, when was the last time you changed your air filter? A clogged filter can really mess things up, creating a rich mixture because the engine isn't getting enough air. Have you noticed any issues with the intake manifold at all?

Thank you for your reply. I haven't checked the oxygen sensor or the air filter yet. Could a faulty oxygen sensor really cause such a pronounced rich mixture? And if the air filter was really clogged, wouldn't I see that immediately?

Well, I am no expert, but a malfunctioning oxygen sensor sends the wrong data to the engine's computer, leading to an incorrect air/fuel mixture. From my experience with my 607, it can definitely cause those symptoms. And regarding the air filter – yeah, you'd think you'd see it, but sometimes it's gradual, and you don't notice until it's really bad. Honestly, if you are experiencing reduced power and rough running, it might be best to take it to a workshop. They can diagnose it properly. A defective injector could cause similar issues.

I appreciate your input. I took it to a workshop, and it turned out to be a defective fuel injector, as suspected. The repair cost was 515€. Thank you for pointing me in the right direction.