What do you do if your car MOT has failed due to large carbonmonoxide emmissions?

Question

do you get a second Garage to check it if you suspect you are being cheated

Answer 1

I know that you really feel vulnerable here, but save yourself some money and time. Don't start throwing parts (pronounced "MONEY") at the car. Pay for the diagnostics to find the cause of the CO failure.
Unless you have some pretty sophisticated and expensive equipment, and you fully understand the chemistry of internal combustion, you aren't going to be able to do it yourself.

CO is partially burned fuel. It can be cause by
loaded air filter
failing fuel injector
failed fuel pressure regulator
MAP or MAF outside standard spec
Sleepy or failed oxygen sensor
Coolant temperature sensor failure
Stuck open thermostat
...and much more.
There is a diagnostic procedure that the tech will follow which is logical, and in the long run will save you money.
The tech doesn't want to stab you for a variety of Reasons, not the least of which he doesn't want to lose his license.
Pay for the diagnostics, and if you aren't satisfied, place a call to your state's regulatory agency and ask them to investigate or give you some insight into th repair estimate based on the diagnostic findings.

Good Luck, and remember, your taxes support that regulatory watchdog, don't be shy about calling them. If the tech is on the up and up, he will welcome the inquiry by the agency (unless the agency wastes a great deal of his time).

Answer 2

You probably have a bad oxygen sensor. Best fix it then go for a retest. It's kind of hard to cheat this test especially for NO unless they tested another car.

Answer 3

Well if you havent replaced your air filter in a while it will greatly help. Carbon monoxide "CO" comes from partially burned fuel, so more air will definitely help.