I have a ‘14 Audi a4 with a p0420 code, this is the only engine code, catalyst effect below threshold. I am worried about dropping $3k on a new catalytic converter and it not resolving the issue, so is there a possibility that it is something else? One of the most common symptoms of a bad cat that I see people talk about is a very distinct rotten egg smell. My exhaust doesn’t smell like that, it smells normal exhaust except very strong. Sitting idling for a while you can smell it around the car, and even in the car. There is no smoke, however the water vapor coming out of the car now that it is cold outside seems to be more water vapor than normal, it looks like a chimney. I’ve replaced the o2 sensor and tried cataclean. The car seems to be running fine and doesn’t seem to be lacking power. This started after I bought a high flow cat, sold the OEM one like an idiot, then bought a used OEM one after realizing I need it to pass my state inspection 🙃 so there is a good chance my used eBay cat is bad. But is a strong exhaust smell (not rotten eggs) black exhaust tips, and lots of vapor a symptom of a bad cat or something else?