Ammonia "cat urine"?

A catalytic converter can smell when it starts to go bad, but that smell is based on hydrogen sulfide, aka "rotten eggs." Most people who complain of an "ammonia" smell find it was due to a grade or brand of gas. Try switching to another grade or brand and see if smell goes away. If you *really* want to get to the bottom of it, take your car to a shop that has an exhaust gas analyzer, it might point to nitrogen as a culprit, versus sulfides, etc. Also, make sure all exhaust related parts are working, like EGR (exhaust gas recirculating), PCV, O2 sensors, no leaks, check for OBD trouble codes.

Yes, the "urine" smell is probably coming from the catalytic converter and is a sulfur (hydrogen sulfide). I would follow the recommendations by KenF, step by step.