Mondy CO emissions

I would check the air filter and maybe give it a little extra air through a mod to an air pipe, as it is so marginal the slightest thing could tip it to a pass.

IIUC the cat will burn up HC rather than CO

Checked air filter already, actually had it oput for the test!!. Modifying an air pipe is a little beyond my expertise and I wouldn't know where to start. Local garage said change cat or plug and oil change. Anyone else with suggestions? Cheers