A Car-ful Passion

Demystifying Your Car's Emission Results

A vehicle's emissions test checks the amount and number of pollutants coming out of the exhaust system. Besides being a requirement in some states, an emission test can also reveal a lot about your car's condition. For this to happen, the tests must be made at different stages of the engine's operation. Here are four stages of engine operation, and what the emission results may mean in each case:


Your engine's load is lightest when it's idle, so it shouldn't produce any alarming emissions. However, if its exhaust gases are overly rich in hydrocarbons and carbon monoxide, then the fuel isn't being burnt up properly and carbon buildup is happening. There are several reasons for this. For example, it may happen if the spark plugs are faulty, the carburetor is misadjusted, or the exhaust valves are burnt.


Your engine doesn't experience a heavy load when you are cruising at a constant speed. High hydrocarbon and carbon monoxide emissions in this state also point to a rich fuel mixture. In this case, however, the fuel imbalance is likely caused by a faulty oxygen sensor, power valve, float level and other engine components involved with fuel mixing.


The engine needs increased power to accelerate, and it gets this from increased fuel/air mixture that is also a little richer than normal. If the car is operating as usual, then the carbon monoxide levels (in the exhaust gases) will be slightly elevated. Again, extremely high carbon monoxide emissions mean that the fuel mixture is too rich even for an accelerating engine.

The hydrocarbons levels, however, should not be elevated. If they are, then your engine's ignition is misfiring, there is a problem with the spark plugs, or the ignition timing is off. All these problems are caused by incomplete combustion.


Deceleration puts your car's engine under light load by reducing fuel flow or making it lean. This also means that both the carbon monoxide and hydrocarbon emissions reduce. If this doesn't happen, for example if the carbon monoxide levels remain high, then there is a problem with your car's deceleration mechanism. This means the engine is receiving too much fuel for its condition; for example, its fuel injector may be leaking.

There are several things you can do to help your car pass a smog inspection. Some of them involve changing the oil, tuning up the engine, and checking the coolant levels. As you can see, these things should not be an issue if you have been servicing your car as required.