If your car has a catalytic converter (all cavaliers from oct 1992) then the most likely problem is the lamda sensor fitted to your exhaust system. This costs over £100 from vauxhall but a universal one from euro car parts is about £35. You will need to change this before your MOT expires as your car will give out higher emissions if the lambda sensor is faulty. If you have an earlier model cavalier and your emissions are ok, it'll probably be a camshaft sensor or something like that. Not really worth spending the money to resolve if the engine is running well enough.
AR
April 2007