Im an MOT tester myself here is some pointers i posted on another site

High HC reading means excessive unburnt fuel could be cause by:

1. Spark plugs need replacing or faulty leads,cap,rotor arm,coil pack check ignition system
2. Faulty CTS ( cold temp sensor )
3 Mechanical fault if present engine will not be running even
4. Lambda sensor
5. Defective air filter

High Lambda reading means the AFR ratio is not correct could be caused by:

1. The engine having a high HC level see above to rectify
2. A leak in the exhaust system even if a minor leak
3. A defective lambda sensor
4. A leak on a vac or intake pipe.

High CO reading can be cause by:

1. All of the above
2. Was the vehicle tested at correct temp was cat allowed to get to operating temp
3. Defective Cat