Toyota iQ exhaust smell rough running

I had something similar happen with my iQ, though I can't remember the exact codes. Rough running and that awful exhaust smell are spot on. Have you checked the exhaust pipe itself for any obvious cracks or damage? Might be worth a quick look before diving too deep into the sensor rabbit hole. I was wondering if it could also be the NOx sensors gone bad.

Thanks! I hadn't thought of cracks in the exhaust pipe, I will take a look. When you had the 'similar' problem, was the engine light on and the smell as strong? Did it happen suddenly or gradually got worse? How did you fix it?

Yeah, the engine light was definitely on, and the smell was pretty strong – almost unbearable at times. It seemed to get worse over a couple of weeks. Honestly, I tried a few things myself, but in the end, I just took it to a workshop. They sorted it out pretty quickly, though. It's probably best to get it checked by a professional, as exhaust issues can be tricky to diagnose properly.

I appreciate the advice. I was hoping it would be a simple fix, but it sounds like a trip to the mechanic is inevitable. I took it in, and it turned out to be, as suspected, a defective lambda sensor. It cost me about 375€, but at least the engine is running smoothly again, and the exhaust smell is gone. Thanks again for your help!