Ok, here is a crash course on emissions [8|]
Firstly to correct or confirm some of the above....
Your car didn't fail on CO2, they don't test CO2, it failed on CO (carbonmonoxide). The limit at fast idle is 0.3% so I'm assuming that the 1.3% is the actual reading that yours is producing.
A cold engine or cold cat will read higher on CO and a failed cat can block (causing power loss) or fail chemically (causing MOT fail) or both. De-cat pipes are virtually guaranteed to result in an MOT fail for reasons explained below.
The "Italian tuneup" is good advice, a good thrash on the road will heat up the cat much more effectively than reving it at the mot station because you are pumping more hot air through the system. As well as getting the cat to a working temperature, if the car does mostly local runs it may also need to burn any contamination off the surface of the cat before it works correctly - this can take some miles to achieve.
The engine management system is designed to re-tune itself whenever the engine is warm and at a steady rpm (idle or cruise). It uses the oxygen sensors (lambda sensors) in the exhaust manifolds to see if it is running rich or weak and then adjusts the fuel up and down to trim the mixture. If this system is working ok the result would be around 0.7-1.0% CO. The catalytic converters job is to scrub these emissions down further by combining the CO, HCs (hydrocarbons or unburnt fuels), and O2 (unburnt oxygen) to create CO2 and H2O. The final result is very low CO and HCs but an increase in CO2 - lesser of the evils!
Given that your CO reading is above 1% it is unlikely that the cat is the sole cause of the fault, though possible as it is only just above this figure. What were the other readings on the fail sheet? In particular I'd be interested to hear what the lambda reading was; this is a calculation of the air to fuel ratio (afr) based on the exhaust gasses. Lambda 1 is the perfect 14.7:1 afr and the normal MOT limits are 0.97-1.03. They use higher and lower limits because artificially lowering the mixture could easily create an MOT pass on CO but it would result in excesive NOx (oxides of nitrogen) emissions due to the weak mixture burning at higher temperatures. NOx is difficult to test for as it tends to occur under load so they include lambda in the test as an easy alternative.
If the cat is entirely to blame for your MOT fail then I would expect the lambda reading to be within limits and possibly slightly high (say 1.00-1.03) but if the lambda reading was slightly low (say 0.95-0.98) then this indicates that the car is running genuinely rich and has a problem aside from the cat.
I'd say that next step would be a good run and retest . If that fails then repost the full results and it would also be useful to know how consistent the readings are - ie are they moving up and down much during the test or staying very stable. With this info we might be able to draw some more conclusions but ultimately you may be in need of some further testing on the car to confirm the fault.
HTH [
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Steve
