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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 require emissions testing for vehicles to be considered legal and roadworthy. 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.
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Table of contents
- What is an emissions test?
- How do emissions tests work?
- What gets checked during an emissions test?
- How to pass an emissions test
- Which states require emissions testing?
- Emissions testing FAQs
What is an emissions test?
An emissions test, also known as a smog 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 gasses 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]
California was the first state to set emissions standards in 1966, though the majority of states now have some form of emission testing requirement.[2] State testing requirements may exclude brand-new passenger cars, but vehicles typically need to undergo emissions testing at vehicle registration renewal time.
How do emissions tests work?
The relatively simple emissions test process may vary from state to state.
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.
How long does an emissions test take?
The 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, only take seconds.
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What gets checked during an emissions test?
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 gasses and pollutants emitted by the vehicle. This may involve one of the following tests:
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On-board diagnostic (OBD) inspection: This plug-and-go test allows an inspector to plug into your vehicle’s data link connector to gather exhaust system data.
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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.
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Two-speed idle (TSI) test: Also called a tailpipe emissions test, the two speed idle test is often reserved for older vehicles. As the name implies, it involves testing a vehicle’s tailpipe emissions while the car idles.[4]
How much does an emissions test cost?
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 (drive-by) option than for going to a testing station and waiting for your test to be conducted.
How to pass an emissions test
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:
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Make sure you have a gas cap (if equipped). A visual gas cap inspection will often be performed to ensure there are no leaks. Be sure to replace your missing gas cap prior to inspection.
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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.
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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.
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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.
What happens if your car fails an emissions test?
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:
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Vapor leak: Usually involving a faulty gas cap seal, a vapor leak will result in a failed emissions test and require repair before retesting.
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HC or CO levels: Your vehicle may emit higher than allowed levels of CO (carbon dioxide) or HC (hydrocarbons). Above-average emissions of either can result in an emissions test failure.
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OBD reader failure: A faulty emissions control system usually causes OBD reader failure, even if the issue isn’t noticeable upon visual inspection.
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Check engine light: In most states, an illuminated warning light results in an automatic failure, regardless of why the light turned on.
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No emissions control equipment: Your vehicle 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.
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Enter your ZIP codeGet My QuotesWhich states require emissions testing?
Emissions testing requirements can vary from state to state, and some states don’t require testing at all. Of those that do, 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.
| State | Requires Testing? | Testing Details |
|---|---|---|
| Alabama | No | N/A |
| Alaska | No | N/A |
| Arizona | Yes, in Phoenix and Tucson | Test every one to two years before vehicle registration or renewal for eligible vehicles. May include OBD and tailpipe inspection. Costs $12.25–$25. |
| Arkansas | No | N/A |
| California | Yes, in Alameda, Butte, Colusa, Contra Costa, Fresno, Glenn, Kern, Kings, Los Angeles, Madera, Marin, Merced, Monterey, Napa, Nevada, Orange, Sacramento, San Benito, San Francisco, San Joaquin, San Luis Obispo, San Mateo, Santa Barbara, Santa Clara, Santa Cruz, Shasta, Solano, Stanislaus, Sutter, Tehama, Tulare, Ventura, Yolo, and Yuba counties | Test for eligible vehicles before vehicle registration or renewal, possibly including visual, functional, OBD, TSI, ASM, and tailpipe emissions inspections. Costs $29.95–$69.95. |
See More: Vehicle Inspection Requirements by State
Emissions testing FAQs
Here are answers to some of the most common questions about emissions testing for vehicles.
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How long is an emissions test good for?
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.
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What do you need to bring for an emissions test?
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.
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What’s the difference between an emissions test and a safety test?
In many states, you need to prove the roadworthiness of your vehicle by completing a required safety test or inspection. This varies from an emissions test, which may also be required. 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.
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How can you check if a car will pass an emissions test?
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.
Sources
- United States Environmental Protection Agency. "Managing Air Quality - Emissions Measurement." Accessed February 23, 2023
- California Air Resources Board. "History." Accessed February 23, 2023
- Air Care Colorado. "How It Works." Accessed February 23, 2023
- Virginia Department of Environmental Quality. "How do I get an inspection?." Accessed February 23, 2023
