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What is an Emissions Test?

By S. Crawford
Updated: May 21, 2024

An emissions test is an assessment of a vehicle’s pollution output under simulated driving conditions. In most cases it can only be performed by auto mechanics with certain specialized equipment and training. Many countries set minimum emissions standards for cars or trucks that are sold within their jurisdictions, and states, provinces, and localities often also require car owners to get tests done every year or every other year in order to be sure that all vehicles using the roads meet these standards. Car engines can emit a range of dangerous gasses as they burn gasoline, and governments have an interest in minimizing these fumes. The test itself is usually pretty easy to perform and usually only takes a few minutes. Results are often automatically reported to government agencies, but falsified tests are nevertheless a problem in some places.

How the Test Is Performed

This test is almost always performed in an auto mechanic’s shop or repair garage. Vehicles are connected to a chassis dynamometer, which is a machine that takes the car though various speed cycles that occur while driving. Cars aren’t usually moving while this happens, though; the test is designed to simulate the regular driving experience without actually performing it. Sometimes the wheels of the vehicle turn on rollers located under them, but the car isn’t actually out on the road.

While the engine is going through its rotations, the dynamometer displays the revolutions per minute (RPM), horsepower, and torque of the engine. An oxygen sensor measures the amount of pollution released by the car during the test. The results are usually saved and compiled into a formal “emissions report,” which mechanics use to determine whether the car passes or fails. In some places, mechanics are required to send an official copy of this report to government emissions offices. Motor vehicle agencies sometimes also receive results digitally, often in real time.

Legal Requirements

At least 15 countries, including the United States, require vehicle emissions tests as a means of limiting and controlling air pollution. The exact testing procedure tends to vary a bit from place to place, and the rules can also be different with respect to how often the tests have to be performed and the levels of toxic emissions that are allowable. Cars will usually fail the test if they emit at levels above the allowable threshold, but what this threshold is, exactly, can change by location. Cars that fail usually have to undergo repairs or else stay off the roads. There may also be a fine levied against drivers whose cars fail.

In the US, the Environmental Protection Agency (EPA) is the federal agency that regulates vehicle emissions. Although this agency does not conduct testing directly, it does set the standards for vehicle emissions. In addition the EPA provides states with federal funding for low emission vehicle incentives and compliance programs. Similar initiatives exist throughout the world as people become increasingly aware of the dangers of vehicle-related pollution.

Dangers of Car Pollution

Cars emit a range of chemicals into the air as a consequence of burning fuel. The most harmful of these include nitrogen oxides, carbon monoxide, volatile organic compounds (VOCs), and fine particulate matter. These pollutants can damage the respiratory systems of people who are constantly exposed to them, and can also harm the environment. Trees, shrubs, and plants that grow near roadways are often stunted or diseased as a direct consequence of pollution, and a range of animals are also impacted. Environmental protection and public health are usually the biggest goals of emissions testing initiatives.

In urban cities with high traffic volume, lower emission volume from cars helps to reduce respiratory conditions. Cars that have fewer fumes emanating from them, either because their engines are more efficient or because they have special emissions filters installed, can help to create a healthier breathing environment for urban populations.

Fraudulent Reporting

The emissions test requirement can be easy to circumvent for people bent on breaking the law. Testers can choose to pass what would otherwise be non-passing vehicles by connecting the testing equipment to a passing car rather than the actual car that needs testing. They might also certify results that have been “doctored.” When either of these things happens, owners are may be able to operate high-polluting vehicles on the roads without penalty.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By lluviaporos — On Sep 13, 2012

@Mor - I think the whole thing is a joke, myself. They have the technology to replace car engines so that they don't have any emissions. They ought to be getting people to do that, rather than just messing around with emissions tests.

Even the cleanest kind of fossil fuel car is still making a lot of pollution. If the government is going to interfere anyway, why not go all the way and solve the problem entirely?

By Mor — On Sep 13, 2012

@Monika - I agree. I mean, when it comes down to it, if you've got a car that's polluting more than usual, you're harming all the people around you. It's not even the same as smoking, because at least with smoking, the harm is isolated and you can make sure you smoke by yourself.

With cars, by their very nature, you're going out in public and adding to the general pollution there. Raising rates of lung disease and cancer and so forth. If someone just decided to go out and spew a bunch of chemicals into the sky for the fun of it, they would be arrested for causing harm. I don't think car drivers should be held to a different standard. And if a car emission test is annoying, the easy solution to that is to take the bus instead.

By JaneAir — On Aug 27, 2012

@betterment - I imagine people who pay for a fraudulent DMV emissions test probably don't think things through that far. They're either short on time, lazy, or just don't have the money to pay to get their car fixed.

Also, I really doubt that people who do the fraudulent tests charge what it would take to actually fix a car. They probably charge way less, which might make that an attractive option for some people.

Either way, we all lose when people do these fraudulent tests, because they result in people driving cars that are polluting the air.

By betterment — On Aug 26, 2012

I had no idea people at emissions test stations would do fraudulent tests! That's terrible. Also, I kind of don't get it. I imagine they charge money "under the table" to the people who they do the fraudulent tests for. Wouldn't it just make more sense for those people to just pay the money to get whatever is wrong with their cars fixed?

By Monika — On Aug 26, 2012

@eidetic - Well, nothing that's done by the government is ever really "free." So would you rather pay for it at the location, or pay for it on your taxes? Either way, you're still going to end up paying money to get emissions testing done on your car.

And I don't really think that's a lot to ask. Everyone benefits from cleaner air because of less pollution due to emissions testing. I for one have no problem paying to get an auto emissions test on my car, or going to get the actual test done. The emissions test centers near me have pretty good hours too.

By eidetic — On Aug 25, 2012

I think mandatory emissions testing is ridiculous. And it's a total waste of my time and money. In my state, it's required, and you have to pay to get it done! So I have to find emissions test locations, take time out of my day to go to one, wait in line, get my car tested, and then pay at the end. I hate doing it, and I think it's unfair the state requires me to do it.

At the very least, it should be free!

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