All petrol models have a MAF, if you tap the cat and it rattles this is a good sign it's gone.

Personally I would have a look at the Lamda though it can't be that bad otherwise the EML would illuminate, spark plugs is a good idea as is the fuel filter which is actually part of the service schedule, however if it was blocked it would do the opposite extreme and run lean.

The main corrective input for the CO is the NTC (Aka CTS) this input is used to determine the base level of fuel injected right from freezing to boiling, if this is giving an incorrect reading I.E colder than what the engine already is then it fueled for accordingly doubt this is the problem, check this last after the above.

Martin