As I understand it Cats convert other gases such as CO and hydrocarbons to CO2 , they don't reduce the level of CO2. So if CO2 is too high it says that levels of the other gases pre cat are too high which indicates a problem with fuel control not the cat surely, i.e. lambda or some other problem.
When my car failed dismally on emissions, the garage said it was probably the Lambda probe which indeed it was because with a new probe it passed easily. I did notice that the CO2 was around 4% or so with the dead probe, a lot higher than your figure seems to be.