S80 NOx Catalyst Aging and Exhaust Leak

Hey there, I have a Volvo S80 too. A similar thing happened to me. I was getting poor fuel efficiency and the check engine light. Turns out, I had a leak in the exhaust system that was messing with the readings from the oxygen sensors, which then impacted the NOx catalyst performance. It's kind of like trying to bake a cake with a hole in your oven – the temperature is never quite right!

Interesting! How did you pinpoint the exhaust leak? And was it an expensive fix? I'm a bit worried about the NOx catalyst itself needing replacement. Is there any way to know whether it's the leak or the catalyst that's the root cause?

For me, the leak was pretty obvious once the mechanic put it on the lift – you could see soot around one of the joints. Given your symptoms and the error code, a leak in the exhaust is a very likely culprit. As for the catalyst, I would suggest taking it to the workshop. Replacing the NOx catalyst can be very expensive, in my case it was only the leak that needed fixing.

Thanks a lot for the advice! I'll take it to my local shop and ask them to check the exhaust for leaks first. Hopefully, it's just that and not a catalyst replacement. I've had them check for leaks and as you suggested, they found a leak! The emissions system has been repaired and the final repair cost was 265€. Thanks again!