Peugeot 407 check engine light

Sounds like a potential air leak in the intake. I had a similar issue with my old Volvo V70. Check your vacuum hoses, especially the one running from the pump to the air valve. Are you experiencing any idle problems?

Idle problems? You mean like, rough idling? Yeah, sometimes it feels a little shaky at stoplights. So you're saying a hose could be the culprit? Where exactly should I be looking, near the air filter?

Precisely. A defective hose connection there can definitely throw things off and trigger the check engine light. Due to the mileage I suggest you bring it to a workshop, they should have a look at the air intake system.

Thanks so much for the tip! Took it to a mechanic, and you were spot on. It was indeed the hose connection from the pump to the valve. Cost me 175€ to fix. Runs like a charm now!