An emissions test is a state-mandated inspection of your vehicle’s exhaust system. Emissions tests play an important role in limiting overall pollution in the U.S. Many states legally require emissions and safety testing for vehicles. Your vehicle may need an emissions test annually or every few years, depending on the state and your vehicle.
Here’s a rundown of how emissions tests work, what to expect from the process, and how to ensure that your vehicle will pass the first time.
An emissions test, also known as a smog test or check, is a diagnostic test that analyzes a vehicle’s exhaust system and the gases and particles it emits. This helps ensure that the vehicle’s emissions don’t exceed certain U.S. and state-specific pollutant thresholds. Vehicle emissions contribute to smog and unsafe levels of greenhouse gases in the air.
It’s important to identify vehicles that emit above-average levels of pollutants and conduct repairs before allowing them back on the road.1
The relatively simple vehicle emissions test process can vary depending on your vehicle type, model year, and state guidelines. Typically, though, an emissions test takes about 20 minutes. Rapid (drive-by) tests, when available, take only seconds.
The first step often includes a visual inspection of your vehicle, looking at the vehicle’s tire pressure, gas cap condition, and any dashboard warning lights. The inspector may also check for the presence of catalytic converters and oxygen sensors, and whether your car emits any smoke through its exhaust system.3
The emissions test comes next, which may involve a gas cap leak test and a diagnostic check of the exhaust emissions. Depending on your state and vehicle, this could be a plug-in diagnostic test, a treadmill-style test to simulate driving conditions, or an idling emissions test. Some states also offer a rapid screen test, which simply involves driving past sensors placed on certain roadways.
In most cases, you’ll pay between $20 and $30 for an emissions test. The cost of an emissions test varies by state and may even depend on the age and size of your vehicle. You may also pay more for a rapid testing option than for going to a testing station and waiting for your test to finish.
Each state determines its own testing criteria, though the process is usually similar from one state to the next. In general, vehicle inspectors look for illuminated check engine lights, visible smoke emissions, and missing components like a gas cap or catalytic converter.
Inspectors also use specialized testing equipment to evaluate the gases and pollutants emitted by the vehicle. This may involve one of the following tests:
On-board diagnostic (OBD) inspection: The OBD emissions test is a plug-and-go test that allows an inspector to plug into your vehicle’s data link connector to gather exhaust system data.
Acceleration simulation mode (ASM) test: Often referred to as a treadmill-style process, this test tracks your vehicle’s emissions while simulating various driving conditions like acceleration and driving up or downhill. It usually lasts just a few minutes.
Two-speed idle (TSI) test: Inspectors often use the two-speed idle test, also called a tailpipe emissions test, for older vehicles. As the name implies, it involves testing a vehicle’s tailpipe emissions while the car idles.4
Emissions testing requirements can vary from state to state, and some states don’t require testing at all. The actual emissions thresholds and testing process may vary by state.
Here’s a look at the states that require emissions testing and what that testing entails.
Passing your emissions test the first time is the best way to save yourself time, money, and energy. While the specific details depend on your vehicle, you can increase your chances of passing the test the first time by doing the following:
Make sure you have a gas cap (if equipped). Your inspector will often perform a visual gas cap inspection to ensure it has no leaks. Be sure to replace your missing gas cap prior to inspection.
Check for illuminated diagnostic lists. In nearly all states, an illuminated check engine light will result in a failed emissions test. Have your vehicle checked out and repaired before wasting your time with an emissions test if you have an illuminated dashboard light.
Get your exhaust system serviced. If you have an older vehicle with a rough idle or smoky emissions, having your system inspected prior to an emissions test can save you from returning for an additional test.
Consider rapid testing options. Some states allow certain drivers to participate in a drive-by emissions test. This test is simpler than the other testing types, and it may be easier to pass.
If you fail your emissions test, you’ll need to perform certain steps to ensure the roadworthiness of your vehicle. Depending on why your car failed, it could need simple or extensive repairs. Then, once you’ve addressed the cause of the failure, you’ll need to retest your vehicle.
Common reasons vehicles fail an emissions test include:
Vapor leak: Usually involving a faulty gas cap seal, a vapor leak will result in a failed emissions test and require repair before retesting.
HC or CO levels: Your vehicle may emit higher-than-allowed levels of CO (carbon monoxide) or HC (hydrocarbons). Above-average emissions of either can result in an emissions test failure.
OBD reader failure: A faulty emissions control system usually causes OBD reader failure, even if the issue isn’t noticeable upon visual inspection.
Check engine light: In most states, an illuminated warning light results in an automatic failure, regardless of why the light turned on.
No emissions control equipment: Your vehicle’s emissions system includes many important components that help control emissions and limit pollutants. If any of this equipment is missing — the gas cap, oxygen sensor, air injection system, or catalytic converter — you’ll likely fail.
The following information can help answer your remaining questions about emissions regulations and testing for your vehicle.
Emissions testing requirements often coincide with a vehicle’s registration, which you’ll likely need to repeat every one to two years. If you have a newer vehicle — or if your vehicle falls under certain exemption categories — this test could be good for even longer.
Depending on the state, you may only need to bring the vehicle to the emissions inspection station. In some cases, however, you might need to bring along your vehicle’s registration or an emissions testing requirement card.
In many states, you need to prove the roadworthiness of your vehicle by completing a required safety test or inspection. This differs from an emissions test, which your state may also require. A safety inspection looks at a vehicle’s safety systems such as its headlights, brakes, horn, windshield wipers, and more to ensure that it’s safe to drive. An emissions inspection looks at the pollutants and gases emitted from the vehicle to ensure that it doesn’t over-emit greenhouse gases.
A vehicle that emits visible exhaust smoke, has an illuminated engine light, or has a missing gas cap or catalytic converter will almost always fail an emissions test. If none of those issues describe your vehicle, it’s more likely that the vehicle will pass. Ultimately, you’ll need to bring the vehicle to an inspection station to see if it passes.
Yes. You’ll typically park your vehicle and leave the engine running for an emissions test.
Yes, in some cases, you can still pass an emissions test with a tire pressure light on. For example, in New York, the inspector will inform you if your tire pressure isn’t in line with the manufacturer’s recommendation, but they won’t fail you for it.5 But it’s a good idea to address a tire pressure light before you take your vehicle in for an emissions test.
No, you shouldn’t do this. If you clear your check engine light on your own, you’ll likely still fail your emissions test. Your vehicle’s computer typically stores a history of diagnostic trouble codes, and the inspector will see you’ve recently tried to clear your check engine light.6 It’s also generally a bad idea not to address vehicle issues before they get worse. You shouldn’t ignore a check engine or other malfunction indicator light.
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