So my08 hatch has failed the MOT on high CO readings

The Garage that did the MOT do not know much about Scooby's

They tested it while the car was hot but made no difference

Tomorrow its going to a different garage for a new cambelt

Any one know how or what to do to get CO down

will the new cam belt cure the problem ?

One thing when I bought the car its got a dump valve on would this cause the problem ?

Any one got any advice

Many thanks