Change your CTS then set your CO to 2-2.5% CO.
It defo sounds like unburnt fuel, when the CTS goes duff a lot of people turn the CO screw down to try to get the fuelling correct, but of course this is not the solution.
The sensor is a 10 minute job to change anyone could do, and only £10 or so from MATT LEWIS 01922 692424.
tabetha