Peugeot 407 Engine Light Lambda Sensor

Considering the age and mileage, and based on the symptoms you've described, a defective lambda sensor is a likely culprit. Have you checked for any error codes beyond the generic 'exhaust treatment malfunction'? Knowing the specific codes could help narrow down the problem. Also, has the car's performance been gradually worsening, or did these symptoms appear suddenly?

Thanks for the quick reply! I did scan for codes; it's throwing a P0420 code, catalyst system efficiency below threshold. Does that confirm it's the lambda sensor, or could it be something else like the particulate filter or even DEF fluid related? I'm not super familiar with the exhaust system in detail.

The P0420 code indicates an issue with the catalytic converter's efficiency. While a faulty lambda sensor can indeed cause this, other components such as the catalytic converter itself, exhaust leaks, or even issues with the engine's air/fuel mixture could also be responsible. Given the complexity and potential for misdiagnosis, I recommend taking it to a reputable workshop for a thorough inspection. They can perform more detailed tests to pinpoint the exact cause. You mentioned that the exhaust system might be leaking. A professional can inspect this in detail. Do not replace expensive parts until the failure mode is confirmed.

Thanks a bunch for all the info and the reality check. I ended up taking it to a local mechanic, and you were right, it was the lambda sensor. The diagnostic tests confirmed it. Cost me 375€ to get it replaced, but at least the engine light is off, and it's running smoothly again. I appreciate the help!