Peugeot 408 Exhaust Flap Issue

Interesting. I had a very similar issue with my 2014 Audi A4, diesel engine. Same symptoms: error code, louder exhaust, and noisy cold starts. In my case, it turned out to be a faulty flap actuator, which controls the exhaust flaps. The actuator motor had failed, preventing the flaps from opening and closing properly. Have you checked the actuator motor itself? You might be able to test it with a multimeter to see if it's getting power.

Thanks for the tip! I'm not super familiar with testing actuators – where exactly is it located, and what kind of readings should I be looking for with the multimeter? Is it something I can easily get to, or is it buried deep in the exhaust system?

On the Audi, it was bolted directly to the exhaust flap housing, relatively accessible after removing some underbody panels. However, the location might be different on your Peugeot. Regarding testing, you should see voltage when the engine is started; the specific voltage range depends on your system. If you're not comfortable with electrical testing, or can't easily access the actuator, I'd strongly recommend taking it to a trusted workshop. A defective exhaust flap can affect engine performance, and further damage can be expensive.

Thanks! I appreciate the insight. Ended up taking it to the shop, just like you said. Turns out the exhaust flap was defective, and the actuator was jammed too. Cost me 190€ to get it sorted, but at least the tractor noises are gone! Cheers!