Cat is not a magic furnace, which burns any junk. The exhaust gases must be fairly clean initially, plus lambda around 1,00 already (0,97-1,03 for example).
No it’s not a furnace nor does it “burn” anything.
Now you (or your mechanic) must do "A-Z" maintenance, go through the full check list. Maintenance tasks/items. Measurements. Adjustments. Diagnostics (OBD2 + oscilloscope + gas analyzer during those checks). "Parts cannon" method failed.
As usual super helpful
I would just add if you feel the need to pull out an oscilloscope when doing car maintenance, you’ve gone way to far and probably need to go take a lay down in a dark room to calm down.
Its from a-z motors brand new
First up has it been serviced, has it had a new air filter recently, any leaks in the intake or exhaust system.
Get a can of carb cleaner for about £5 and clean up the throttle body and any intake pipes.
There is usually a crank case breather that vents into the air intake system and it this is pushing out a lot of oil vapour then it can clog up the intake side of things and fowl the throttle body.
Take the plugs out make sure they are gaped correctly that the HT leads are in a good condition and correctly fitted to the coils.
look up on YouTube how to test your O2 sensor, you will need a multimeter, and you can get one on Amazon like this for less than a tenner which will do everything you need it to do
https://amzn.to/3PZW6qQ
This will tell you if the O2 sensor is working properly and if not then the engine could be putting in more fuel than is needed leading to incomplete combustion, higher CO higher hydrocarbons.
If it is only marginally outside of spec, then it may need an “Italian tune up” especially if the car used to belong to someone who only ever drove it to the bingo once a month at 20 mph. “An Italian tune up” is code for take it down your nearest bypass screaming the nuts out of the engine in 3rd or 4th gear and not changing up, taking down some country lanes in a “spirited fashion” basically thrash the hell out of it for a few miles, no car likes to be left standing and they like to be driven hard especially these little 1,2 engines they love to be rev’d
In Italy if the car isn’t constantly bouncing off the Rev limiter you’re not driving it properly.
If you are 100% confident you’ve not got a dodgy CAT then these are the things i’d be doing.
If you have a gas analyser then you probably wouldn’t be here and if you have an oscilloscope that that’s a clear signal you have no friends.