Peugeot 1007 NOx Catalyst Exhaust Leak

I've got a Peugeot 1007 as well, and your story sounds familiar. NOx – that's Nitrogen Oxides, nasty emissions stuff. The NOx catalyst is part of your emissions system, basically a catalytic converter that's specifically designed to reduce NOx emissions. A leak upstream will definitely throw things off, confusing the sensors which can turn on the engine light. Where exactly do you think the exhaust is leaking from?

The leak seems to be coming from somewhere around the catalytic converter area. I'm just not sure if it's a simple fix like a cracked pipe or something more serious with the NOx catalyst itself.

Okay, if it's around the catalytic converter, it really could be a cracked pipe or a dodgy gasket. But if the diagnostics are pointing towards the NOx catalyst specifically aging, and the car's got a fair amount of mileage, then it might be time to start thinking about replacing the whole unit - especially if that leak has been there for a while. Those leaks mess with the readings and cause the catalyst to work harder than it should causing it to prematurely wear. Best bet might be to get it checked out at a trusted workshop. They'll be able to pinpoint the leak and assess the state of the catalyst properly.

Thanks so much for the advice! I took it to a local workshop as you suggested. Turns out, a small crack in the exhaust manifold was causing the leak, and it damaged the NOx catalyst. They replaced the manifold and catalyst for 265€. Hopefully, this sorts it out for good!