This weekend the car went in for its MOT and low and behold it failed on emissions by a very large amount. CO was 10x what it should be at 2.20% and Hydrocarbons were more than double at 500ppm. This was kind of expected but not with these results as before this the car had been burning oil through the PCV system but i have since fitted an oil catch can and that seems to have solved that problem as absolutely no smoke comes out the exhaust now bar steam when the engine is cold.

The garage was not very useful with providing a solution as they, naturally, wanted me to leave it at the garage to be fixed. Currently my theory is that the car is running very rich because of a lack of air getting into the combustion chambers as I've already tested the front heated lambda sensor and the voltage was within the range it should be, I've also taken a look inside the catalytic converter and that's fairly clean. I assume that if the fuel mixture is rich than it must be on the air side of the ratio as the car isn't putting out any black smoke which it apparently would be if the fuel was rich on the petrol side of things.

Anyway my main question is: What would be the best way to diagnose if the fuel mixture is rich or not and if it is, where is the best starting point on diagnosing the which part is at fault?

Just as a side note, there are no diagnostic codes stored in the car, but I do have a check engine light on and a "Reduced Engine Performance" note on the dashboards display. I also made a post about whether it was the catalytic converter here: www.reddit.com/r/mechanics/comments/789f5b if that helps