well, allegedly they remove all the nasty stuff from the exhaust, like carbon monoxide, Nitrogen dioxide and Hydrocarbons...hence the term '3 way catalytic converter'..and turn it into not so nasty stuff. The MOT test will measure Carbon Monoxide and HC's (hydrocarbons....essentially partially or unburnt fuel), so not everything that the cat should deal with. In short, the MOT standards are less rigorous vs the test manufacturers have to meet for type approval. I think it's all BS. Not that reducing emissions is a bad thing, but you set up very specific tests that manufacturers will have to pass..and they will design their vehicles to pass them. I've seen air fuel ratio plots where the fuel ratio leans right out at a certain RPM. Right at where the emissions are checked to pass regulations. Coincidence? the most extreme version of this was the VW emissions scandal. I guess my argument is that the vehicles are designed to pass a specific test and not reduce real world emissions under normal driving/riding conditions. There was one very interesting article where a reporters dad was thinking of changing his old car for a newer car almost purely based on emissions and 'doing the right thing'. They were able to get an emission tester (and it was tough...most suppliers didn't want to lend kit as they said 'our main customers are the manufacturers and this could really upset them'). This thing literally had to be strapped to the back of the car and collected everything from the exhaust, so almost a closed system. They tested the dads 10 year old car and then a brand new version of the same car, several euro levels higher. They drove the same route, so stop start, town and faster roads...so a real world test..for both cars. Surprisingly, the older car was cleaner than the new car. I've not seen the test repeated, but I'd be very interested if it was. This all just seems to be giving lip service to emissions, brow beating us into accepting ever tighter regulations and pointing to flawed tests that 'prove' the emissions are lower, while precious metals are being mined, consumerism is king and this is all meant to protect the planet when in reality it's not...it actually could be the opposite
I'll get off me soap box now!