When a vehicle fails an emissions test it is the vehicle that has failed, NOT the cat! The purpose of a cat is to convert harmful exhaust gases into less toxic substances before they leave the exhaust, not to enable a badly running vehicle to pass an emissions test. Often fitting a new cat will enable the vehicle to pass an emissions test, but this is only a temporary solution as the badly running vehicle will soon overpower the new cat, and the same situation will occur at the next MOT. Before you blame the cat remember that the cat can only convert what the vehicle is putting into it. If the fault on the vehicle is fixed and has been caught in time, it may pass an emissions test with the same cat.
1. Make sure the car has been run up to temperature before the emissions test is carried out, and also ensure the lambda is tested. If these points are not followed the test will provide false readings, and be of no use to the technician. What section of the emissions test has the car failed?