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Why Emissions Testing Is a Good Thing

Exhaust smoke

Emissions testing is a part of car ownership in many areas of the United States. Some states mandate testing regularly for vehicles to gain licencing to drive on public roads. For those who own an older vehicle or one that is high in mileage, emissions testing can be a serious headache. Yet even if you live where emissions testing isn't required, it may be a good idea to have it done occasionally regardless.

What? Do something that's not required by law? What for?

An emissions test tells you a lot about your vehicle and for the relatively small fee, it can save you a lot of diagnostic time down the line.

Most people believe that emissions testing is all about pollution. It is, in part, but for the mechanic, it's also an important indicator of how your engine is doing. An emissions result that shows high amounts of some pollutants could indicate something is wrong. High soot levels could mean you have a lot of buildup in your engine or exhaust. High NOx or other gases may mean your engine is not performing as well as it could and might be costing you money at the pump.

Here are a few common emissions test findings and what they might mean. Be aware that some problems will manifest only under "loaded mode" (dynamo) testing and others might be more prevalent in an idle test. Also be aware that the most common reason for failure in a loaded mode test is an EVAP system failure, so check the gas cap. Vacuum leaks are also a common failure point in either emissions test method.

High HC Emissions

If your test shows elevated hydrocarbon (HC) emissions, you likely have fouled spark plugs, a bad plug in the line, burned exhaust valves, an intermittent misfire (usually due to a lean or rich condition), or fuel injector problems.

High CO2 Emissions

This is almost always an over-rich fuel mix, especially on carbureted vehicles. Check the float, power valve, and choke. Newer fuel injected vehicles should be throwing a code, but if not, check fuel injection controls, injectors, and O2 sensors.

High NOx

High nitrogen oxide (NOx) emissions are usually an exhaust gas recirculation (EGR) problem. We will talk about EGR systems and repairs in an article very soon.