Peugeot 406 running lean

I sympathize. I had a similar issue with my old Opel Vectra diesel – reduced power, rough running, and the check engine light. It was a faulty mass airflow sensor throwing off the fuel mixture. Did you check this sensor?

I haven't examined the mass airflow sensor. Wouldn't a malfunctioning sensor trigger a specific error code? I'm only getting a generic "lean mixture" code. Could the mass airflow sensor cause a lean mixture without a dedicated error code?

That's a valid point about the error code. My Vectra did throw a specific code, but I've heard of less precise errors. Since you're only seeing a lean mixture code and have checked for vacuum leaks, pinpointing the cause is challenging. I'd suggest a thorough inspection at a reputable workshop with sophisticated tools to evaluate the catalytic converter and related systems.

I took the Peugeot to a workshop for diagnostics. It was a defective fuel injector causing the lean running. The repair, including injector replacement and labor, was 515€. Thanks for guiding me to a resolution.