CO isn't a cat issue. Nox and HC would be a cat issue. Service it, new plugs and filters inc fuel, oil etc put a tank of branded super in it, maybe some injector cleaning snake oil too. Thrash it about a bit to clear it all out and take it back for a retest red hot. It's basically it's running a tad rich (too much fuel), but only slightly, so not enough to send your HC over the limit, do you have any mods? The above is to try an increase combustion effiency to get that wee bit of spare fuel burnt and bring the co down.