Catalyst test 1. The exhaust emission test The catalyst test is part of the MOT test for most class IV spark ignition petrol engined passenger cars with four or more wheels first used on and after 1 August 1992. Carbon monoxide (CO), hydrocarbons (HC) and lambda (λ) are checked at fast idle speed and carbon monoxide (CO) is checked again at idle speed. The test should be self explanatory using the automated routine on 1996 specification exhaust gas analysers. The following points should be borne in mind