Megane Exhaust Treatment Malfunction

That sounds rough! I had a similar saga with my Megane. The check engine light was on, and the smell was awful. From what you're describing, it does sound like an exhaust leak. Have you checked the lambda sensors? I had a similar issue and I had to replace one of the NOx sensors as well.

Lambda sensors, eh? I haven't looked at those specifically. Where are they located, and is there any easy way to test them without specialist equipment? I'm trying to avoid a costly workshop visit if possible!

They're usually located in the exhaust system, before and after the catalytic converter. Honestly, testing them properly without the right gear is tricky. Given all the symptoms you're experiencing – the light, the smell, and the rough running – it might be best to get it properly diagnosed. It could be something simple, but it could also be causing further damage if left unchecked. In my case the cost was 375€.

Thanks for the advice! I've booked it in at the local workshop. They confirmed it's a defective lambda sensor that needs replacing. Total cost will be around 375€ including labour. Hopefully, this sorts it out!