Emissions testing for petrol engines appears complex but once the date of first use and the vehicle type has been established then it's really quite simple.

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

Basic fault finding for advanced emission test failures:

1. If a vehicle fails for high CO at idle and /or fast idle then.... if Lambda is too low, the mixture is too rich. This can be caused either by a misfire or by a faulty Lambda sensor. If it's running very rich, the 0₂ and HC will be higher than normal.
2. if it has a high Lambda and high 0₂ (0.5 to 1.5%), the "CAT" could be faulty. Before condemning it make sure its hot enough to work, the Lambda and other sensors are working OK and the exhaust downpipe hasn't got any leaks.
3. if the vehicle fails for high HC then ...if the CO passes, the vehicle is probably misfiring. Or if the CO fails and the Lambda reading is too low, the mixture is probably too rich and it may also be misfiring.
4. if the CO fails, the Lambda reading is high and the O₂ reading is high (0.5 to 1.5%) the "CAT" could be faulty. Again check out the simple measures above before condemning it.
5. if the Lambda fails then...if its too high the Lambda sensor may be faulty, the mixture may be too weak or there may be a hole in the exhaust downpipe.  If Lambda is too low the sensor may be faulty, the mixture may be rich or the vehicle may be misfiring.