Two cars driving on a road

If it’s time to renew your car insurance or vehicle registration, your vehicle could also be up for an emissions test or smog check. Why do you have to do this? In 1990, the Federal Clean Air Act was amended in an effort to greatly reduce air pollution. As a result, the Environmental Protection Agency devised a set of emissions standards to minimize the amount of hazardous air pollutants released by motor vehicles.1 This means your car may have to undergo periodic testing to ensure it’s within EPA standards and is limiting its negative impact on the environment.

Is vehicle emission testing required?

As of today, more than 30 states require some level of emissions testing by law. To what degree your vehicle needs tested varies by state, and in some areas, by county. Check your state’s EPA resources to see if and what kind of testing is required for your vehicle.

Testing procedure

Testing procedures gauge your car’s emissions and ability to track the pollutants it releases. In particular, a test can potentially review the levels of carbon dioxide (CO2), hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx) and other emissions coming from your car.2

Depending on your state’s process, testing can take 15 to 30 minutes. Although there are a variety of inspections that can check your vehicle’s emissions, the following are the most common types.

On-Board Diagnostic (OBD) inspection

For model year 1996 and later vehicles, the OBD inspection is the most conventional test.3 This test verifies the data from your car’s internal emissions diagnostics system and ensures that your Malfunction Indicator Light (MIL), the “check engine” light, is working properly.

Acceleration Simulation Mode (ASM) test

Many cars made in 1995 or earlier don’t have an OBD system installed, so they require a different method to test their emissions, such as an ASM test.4 Using a dynamometer — similar to a treadmill — and a tailpipe sensor, these tests simulate driving conditions to measure your car’s emissions.

Two-Speed Idle (TSI) test

A TSI test is generally reserved for older cars. It tracks exhaust emissions when the engine is idling at a high and a low speed.4

What happens if you pass the test

Immediately following your test, you should receive the results. If you pass, your car can be legally registered and can hit the road. Keep track of any paperwork you receive, and if needed, find out when you might need to take another test by contacting your state’s motor vehicle agency.

What happens if you fail the test

In the event your vehicle fails the test, you cannot drive your vehicle until you make the necessary repairs and pass another test.

The results of your failed test should give you an idea of what fixes need to be made. Share your results with your mechanic to understand your repair options. Some of the common reasons why vehicles fail, according to Maryland’s Motor Vehicle Administration include:

Excessive HC or CO levels

Your car is emitting HC or CO above established standards.

Sample dilution failure

A valid exhaust sample couldn’t be taken, usually due to a leak in the exhaust system or poor engine adjustments.

OBD failure

Either the OBD emissions control components or the dashboard’s MIL are malfunctioning.

Emissions control equipment absent

The equipment necessary within the vehicle to test emissions is absent or has been disconnected.

Loose gas cap

Your gas cap is not creating a tight seal and could be leaking vapors. This is common in older vehicles with worn-out gas caps.

The majority of issues that lead to a failed test can be remedied with regular maintenance.5 Consult your vehicle’s manual to see how often you need to change your oil and when you should consider a diagnostic test. If you have an emissions test coming up, have your vehicle inspected by a mechanic beforehand. Thinking ahead can sometimes make all the difference between passing and failing an emissions test.

Being prepared for an emissions test can help keep your car on the road. And Nationwide can help keep you and your car well-protected. Learn more about your car insurance coverage options.


The information included is designed for informational purposes only. It is not legal, tax, financial or any other sort of advice, nor is it a substitute for such advice. The information may not apply to your specific situation. We have tried to make sure the information is accurate, but it could be outdated or even inaccurate in parts. It is the reader’s responsibility to comply with any applicable local, state, or federal regulations. Nationwide Mutual Insurance Company, its affiliates and their employees make no warranties about the information nor guarantee of results, and they assume no liability in connection with the information provided.