HCs unburned fuel....either too much of it, or it is not getting lit. A feedback EFI system may interpret this as bein too rich, and compensate by further leaning an already lean mixture, making things worse, and potentialy ruining a piston or head in the long run. HCs also evaporate out from your oil, and show up in the test. This is much less pronounced if you have new oil for the test. Bad rings will put out some HCs that you can see come out the pipe too.

CO is the result of overfueling or lack of air(dirty air cleaner/something blocking intake)

CO2 is a normal byproduct of petroleum combustion. If it goes down, that usualy means CO went up.

O2 is normaly present in exhaust, and used by the computer to calculate mixture. A misfire leaves excess O2 in the throws that whole system out of whack. too much O2 in the reading will cause the smog machine to invalidate the test and usualy is an exhaust leak. Often when this happens, the shop will try to sell you an exhaust system. THIS IS WRONG, the sensor may be inserted in ANY hole in the exhaust DOWNSTREAM OF THE CAT. You can punch a hole in the pipe just before the muffler just for this purpose.

NoX is nitrogen which has burned at too high of a temperature. This happens from a lean mixture, advanced timing, or a defective EGR system. Generaly there is an inverse relationship between HCs and NoX.


The MS system has no provision for EGR, AIR or the stock MIL(Malfunction Indicator Lamp. It also has no CARB exemption. Those are three reasons you will automaticaly fail in california before the test even starts.

The Cat requires the AIR to function effectively on most cars, and a defective cat will cause out of limit emissions even if nothing else is wrong with the car. IF the cat is at full operating temperature, your car can pass, even if it would normaly fail. If your engine and tune are perfect, you can pass the tailpipe test to new car standards without the cat testing in steady RPM conditions. I don't think you can pass the dyno test though.

This is an oversimplified summary, and the complex interrelationships between the various emissions and their causes has filled many volumes.


BASIC EMISSION TEST CHECKLIST:
1) clean air filter
2) new spark plugs and good wires
3) new oil
4) unobstruced flame trap/PCV system(especialy on Volvos)
5) Visual undrehood inspection for cracked, missing, or disconnected vacuum hoses
6) Proper preconditioning.....I can't stress this one enough!
Drive on the highway for at least 20 minutes before the test, then
Don't turn off the car untill the test is run.

If you follow these steps, most old beaters will pass. If you dont, especialy #6, a newer car with no real problem will fail. If you test station does not have a 'no pass no pay' policy, they HAVE AN INCENTIVE TO FAIL YOU, and an easy legal means of doing it. I have seen this a lot, the shop typicaly makes $75 to $500 per repair, depending what they say the problem was, and they don't even have to fix anyting.