It's a common misconception that cats reduce emissions! They only work after the engine has been running at high revs for 20+ min that is why your engine has to be run for ages and has to hold high revs for the cat test for the Ministry Of Transport annual test.
What has reduced emissions is the Oxegen sensor in teh exhaust pipe and emissions control in the ECU not the cat.
Cats will reduce emissions when running on the motorway but when stuck in traffic in town or cold they increase the CO2 and general pollutants released into the atmosphere by up to 10%
A well sorted engine like the Honda Civic never needed a Cat at all and I would hazard a guess that the modern SAAB engine would not need one either.
I do not normally believe in Conspiracy theorists but as each and every CAT you buy a royalty of XX% goes to one person who patented the process.
This same individual is extremely lavish with funding political parties (those that sponsor legislation to make cats compulsory) etc. I know a lot more is out there and provable but I will not rant on about it here!
Politicos don’t we love them all??
Has the car been running fast or slow before stopping?
I would expect either high Sulfur fuel or poisoned CAT.
Running on LPG also can cause some “pong” though I do not think the LPG is responsible for poisoning the CAT.
Leaded Petrol will do it every time! I know that if a tank at the garage / gas station has had leaded in it for many years the lead can be deposited on the inside of the tanks, so if they fill with Unleaded and sell it as such you can still get a poisoned Cat!
I had to inspect storage tanks used since 2nd world War, for aviation fuel, and the Lead additives had built up fantastic crystal structures 4 feet high! (I had to wear a full environmental suit and be lowered 75 feet into the underground silo through an 18” opening. I earned my wages that day!)