Failed MOT emissions

I have a 1999 1.8 petrol 129k and it has failed its mot on the CO emissions at idle reading 0.5 limit 0.3. recently serviced only thing i didn't change was the plugs. I put new plugs in and put in a fuel additive still failed same limit any ideas would be appreciated also is there away to manually adjust the mixture or is it all done automatically.

Cheers.

The vital clue here is the Lambda reading which you should have on the emissions print-out. Lambda is a calculation of the Oxygen content of the exhaust gas leaving the engine and is unaffected by the Catalytic converter. An ideal reading would be 1.0 and would indicate that the catalytic converter is unable to Oxydise the CO content of the exhaust gases and is therefore faulty. However a reading of less than 1.0 e.g. 0.8 would show that the engine is producing high levels of CO which the catalyst is unable to deal with effectively and attention should be paid to the operation of the engine management system: Oxygen sensor dirty/ high fuel pressure/ blocked air filter etc. I shouldn't worry about the spark plugs as they would, if faulty cause a misfire which would actually reduce CO emissions but increase HC emissions. D..