Hi Shaun

I would say you have more than a cat problem

If the cat has failed I would expect CO to be around 0.5% and HC around 50-100 PPM

The cat may well be dead but I think you have another problem that needs fixing first, rich mixture, misfire etc. Once you have fixed, I would do a pre cat gas test, if the CO is down to 0.5% (or abouts) and the same at the tail pipe, then fit a new cat

I may stand to be corrected, but thats my thinking

Peter