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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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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]

Good to Know

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:

  • 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.

  • 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: 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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:

  • 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 dioxide) 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 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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Which 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.

StateRequires Testing?Testing Details
ArizonaYes, in Phoenix and TucsonTest every one to two years before vehicle registration or renewal for eligible vehicles. May include OBD and tailpipe inspection. Costs $12.25–$25.
CaliforniaYes, 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 countiesTest for eligible vehicles before vehicle registration or renewal, possibly including visual, functional, OBD, TSI, ASM, and tailpipe emissions inspections. Costs $29.95–$69.95.
ColoradoYes, in Boulder, Broomfield, Denver, Douglas, and Jefferson counties, as well as portions of Adama, Arapahoe, Larimer, and Weld countiesTest every one to five years before vehicle registration or renewal for eligible vehicles. May include an OBD, TSI, ASM, RapidScreen, and gas cap test. Costs $15–$25.
ConnecticutYesTest every two years for eligible vehicles. May include an OBD, TSI, opacity, and gas cap pressure leak test. Costs $20.
DelawareYesTest every other year for eligible vehicles. May include an OBD, TSI, idle curb test, and gas cap test. No charge.
GeorgiaYes, in the Atlanta metropolitan area, including Cherokee, Clayton, Cobb. Coweta, DeKalb, Douglas, Fayette, Forsyth, Fulton, Gwinnett, Henry, Paulding, and Rockdale countiesTest every year before the vehicle registration date for eligible vehicles. May include an OBD, fuel cap, and visual inspections. Costs up to $25.
IdahoYes, in Kuna and Canyon countiesTest every two years for eligible vehicles. May include an OBD, TSI, and gas cap inspections. Costs $14.
IllinoisYes, in Cook, Dupage, and Lake counties, as well as certain ZIP codes in Kane, Kendall, Madison, McHenry, St. Clair, and Will countiesTest every two years for eligible vehicles. Will include an OBD inspection test. Costs $20.
IndianaYes, in Lake and Porter counties onlyTest every other year for eligible vehicles. May include an OBD, single idle speed, and gas cap tests. No charge.
LouisianaYes, in Ascension, East Baton Rouge, Iberville, Livingston, and West Baton Rouge parishesAnnual emissions and safety inspection for eligible vehicles. May include visual, gas cap, and OBD inspection. Costs $18.
MaineYes, in Cumberland CountyTest every year for eligible vehicles. May include OBD, gas cap, and safety inspections. Costs $12.50–$18.50.
MarylandYes, in Anne Arundel, Baltimore, Calvert, Carroll, Cecil, Charles, Frederick, Harford, Howard, Montgomery, Prince George’s, Queen Anne’s, and Washington counties, as well as Baltimore CityTest every two years for eligible vehicles. May include OBD, gas cap, opacity, and tailpipe tests. Costs $10–$15.
MassachusettsYesAnnual emissions and safety test for eligible vehicles. May include an OBD or exhaust opacity inspection. Costs $35.
MissouriYes, in St. Louis City, St. Louis County, St. Charles County, and Jefferson CountyTest every other year for eligible vehicles before vehicle registration or renewal. Will include OBD and safety inspections. Costs $36.
NevadaYes, in portions of Clark and Washoe countiesTest every year for eligible vehicles. May include visual, opacity, OBD, and TSI inspections. Costs $62–$99.
New HampshireYesTest every year around registration for eligible vehicles. May include visual, mechanical, and OBD inspection. Costs $20–$50.
New JerseyYesTest every two years for eligible vehicles. May include OBD and safety inspections. No charge at New Jersey state testing facilities.
New MexicoYes, in Bernalillo CountyTest every two years for eligible vehicles. May include visual, pollution control equipment, OBD, and tailpipe inspections. Costs $15–$25.
New YorkYesTest every year for eligible vehicles before vehicle registration or renewal. May include safety, mechanical, opacity, and OBD inspections. Costs $11–$27.
North CarolinaYes, in Alamance, Buncombe, Cabarrus, Cumberland, Davidson, Durham, Forsyth, Franklin, Gaston, Guilford, Iredell, Johnston, Lincoln, Mecklenburg, New Hanover, Randolph, Rowan, Wake, and Union countiesTest every year for eligible vehicles. May include OBD, visual, and safety inspections. Costs $30.
North DakotaNoN/A
OhioYes, in Cuyahoga, Geauga, Lake, Lorain, Medina, Portage, and Summit countiesTest every two years for eligible vehicles. May include an OBD and gas cap inspection. No charge for the first three tests. If needed, a fourth test costs $18.
OregonYes, in Portland Metro and Medford/Ashland areaTest every two years for eligible vehicles before vehicle registration. May include a tailpipe or OBD inspection. Costs $20–$25.
PennsylvaniaYes, in Allegheny, Beaver, Berks, Blair, Bucks, Cambria, Centre, Chester, Cumberland, Dauphin, Delaware, Erie, Lackawanna, Lancaster, Lebanon, Lehigh, Luzerne, Lycoming, Mercer, Montgomery, Northampton, Philadelphia, Washington, Westmoreland, and York countiesTest every year for eligible vehicles. May include visual, OBD, TSI, idle, and gas cap inspections. Costs vary by inspector.
Rhode IslandYesTest every one to two years for eligible vehicles. May include OBD and safety inspections. Costs $11–$15.
South CarolinaNoN/A
South DakotaNoN/A
TexasYes, in Brazoria, Collin, Dallas, Denton, El Paso, Ellis, Fort Bend, Galveston, Harris, Johnson, Kaufman, Montgomery, Parker, Rockwall, Tarrant, Travis, and Williamson countiesTest every year for eligible vehicles. Will include OBD and safety inspections. Costs $11.50–$25.50. 
UtahYes, in Cache, Davis, Salt Lake, Utah, and Weber countiesTest every two years for eligible vehicles. May include visual, mechanical, and OBD inspections. Costs $20–$30.
VermontYesTest every year for eligible vehicles. Will include OBD, visual, and safety inspections. Costs $2.21 plus a $6 inspection sticker fee.
VirginiaYes, in the counties of Arlington, Fairfax, Loudoun, Prince William, and Stafford, as well as the cities of Alexandria, Fairfax, Falls Church, Manassas, and Manassas ParkTest every two years for eligible vehicles before vehicle registration or renewal. May include on-road, visual, ASM, and OBD inspections. Costs up to $28.
West VirginiaNoN/A
WisconsinYes, in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington, and Waukesha countiesTest every other year for eligible vehicles. Will include a visual and OBD inspection. No charge.

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.

  • 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.

  • 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.

  • 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.

  • 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.


  1. United States Environmental Protection Agency. "Managing Air Quality - Emissions Measurement." Accessed February 23, 2023
  2. California Air Resources Board. "History." Accessed February 23, 2023
  3. Air Care Colorado. "How It Works." Accessed February 23, 2023
  4. Virginia Department of Environmental Quality. "How do I get an inspection?." Accessed February 23, 2023
Stephanie Colestock
Stephanie Colestock

Stephanie is a DC-based freelance writer. She primarily covers personal finance topics such as insurance, loans, real estate investing, and retirement. Her work can be found on CBS, FOX Business, MSN, Yahoo! Finance, Business Insider, and more. When she isn't helping people plan for their financial futures, she is traveling, hiking with her kids, or writing for her own website, She can be reached on Twitter @stephcolestock