there is set limits for emissions from cars, a bit of a pain to find exact figure rather than just proceedures but here is some general information which is a good starting point. taken from the second web link
http://www.motuk.co.uk/manual_730.htmhttp://www.bobmckay.co.uk/p_emissions.htmlFrom 1st August 2001 a simplified emissions check was introduced and this will be carried out on vehicles prior to doing a full "CAT" test. If the vehicle meets the requirements during this BETs (basic emission test) then it passes. There will be no need to measure the engine temperature using the analyser probe but the vehicle must be at normal running temperature. However, engine rpm will still be measured. If the vehicle fails the BET then the full "CAT" test is applied.
The BET standards are:
Fast idle 2500-3000rpm
CO no more than 0.3%
HC no more than 200 parts per million
Lambda between 0.97 and 1.03
Normal idle 450-1500rpm
CO no more than 0.5%
# 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.
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.