Passenger cars:
First used before 1/8/1975 - Visual test only. Failure for excess idle speed, dense blue or black smoke for 5 seconds at idle or during acceleration up to 2500rpm or half engine max speed whichever lowest.
First used between 1/8/75 and 31/7/86 - meter test CO <=4.5%, HC<=1200ppm.
First used between 1/8/86 and 31/7/92 - meter test CO<=3.5%, HC<=1200ppm.
First used between 1/8/92 and 31/7/94 - advanced emission test (CAT) if its running on petrol when presented, to the vehicle specific limits usually around CO<=0.3%, HC<=200ppm and Lambda 0.97 - 1.03. Remember from 1st August 2001 a basic emission test will be carried out initially.
Note that the tests vehicles for first registered on or after 1/8/92 require expensive exhaust gas analyser equipment which is usually only available to the trade. However, there is DIY equipment available which is relatively inexpensive to enable emissions tests on vehicles prior to that date.
Vehicles with 6 or more seats are not passenger cars and are subject to less stringent requirements usually:
Registered between 1/8/92 and 31/7/94 - meter test CO<=3.5% HC<=1200ppm
Registered on or after 1/8/94 and an exact match can be found in the DTP data book or analyser database then CAT test to specified limits otherwise CO<=3.5% HC<=1200ppm