UK MOT Emissions Failure

[Zino986]

Hi,
My 1998 Boxster 2.5 has failed the emissions part of the UK MOT test, but it was listed on the emissions report as a 2.7
Question is, would this make a difference to the parameters that it should be within?
The only thing it failed on is the CO reading (approx. 1.4%), the expected result max was 0.20%
It has a single O2 sensor, so I cant, (or don't know how to) check the operation of the cat.
Can anyone point me in the right direction?

By-the-way, it is used daily 50mls, not garaged, and have also used cataclean and 97ron fuel before the test.

Thanks in advance