CO Failure can mean several things, but is most likely caused by a poorly running vehicle. If a cat becomes
contaminated with unburned fuel, it will become unable to convert the gases that pass through it. A new cat
running at full capacity will almost certainly lead to an MOT pass, but unless the fault is rectified, the customer will return the next year complaining of the same problem.
HC Failure is an indicator that the vehicle is over-fueling. A vehicle will fail on this reading if it has HCs in excess of 200ppm. HCs in excess of 60ppm indicates a vehicle problem that needs rectifying as soon as possible.
Lambda Failure means there is a problem with the air:fuel ratio on the vehicle. If the lambda reading is less than 0.97 the vehicle is running too rich. If the lambda reading is greater than 1.03 the vehicle is running too lean.
The table below contains the European Emission Standards for passenger cars, defining the acceptable limits for exhaust emissions of new vehicles sold in EU member states.