For most people, smog checks are an inevitable part of life - like death and taxes. The emissions inspections can sometimes be intimidating, especially if you don't know how the process works or whether your car will pass the first time.

What's a Smog Check?

Smog checks, or emissions tests, are state-mandated inspections designed to minimize vehicle-generated pollution. In most locations (but not all), you must have a smog test annually or bi-annually.

Smog checks hold older cars to different standards.

Types of Smog Tests

There are two types of smog tests: tailpipe and onboard diagnostics (OBD). Although both are designed to measure a vehicle's emissions, they're conducted differently.

Onboard diagnostics

Vehicles built after 1996 are equipped with a standardized set of onboard diagnostics called OBD-II. The technology uses a computer and sensors to detect when a car has problems that could lead to increased tailpipe emissions.

Smog tests are usually pretty simple on vehicles equipped with OBD-II. Most testing stations connect a scan tool to the car. The tool communicates with the vehicle's onboard computer to check for active or pending diagnostic trouble codes (DTCs).

If there are active DTCs, the check engine light (CEL) will pop on. And that's usually an automatic emissions test failure. Pending codes, on the other hand, do not usually trigger the CEL. But they will typically result in a failed emissions test.

Also, during the OBD II check, the station will verify whether all the car's readiness monitors have run and passed. The monitors are a series of diagnostic checks conducted by the vehicle's computer. If everything within a particular part of the OBD system works properly, the computer will flag that portion of its test regiment as "passed". A "not ready" reading will be displayed if the computer has not yet run a certain test.


Older vehicles are subjected to a more invasive tailpipe test. The process involves inserting a gas analyzer into the car's tailpipe to check for excessive emissions. Some states also require OBD-II-equipped cars to undergo a tailpipe test.

OBD and tailpipe tests both include a visual inspection as well. During this portion of the assessment, the car is checked for visible issues, such as gas cap problems and modifications to emissions equipment.

Note: Smog testing varies by state and county. Check with your local Department of Motor Vehicles (DMV) to get the proper information for your area.

Why might your car fail?

Cars that have OBD-II will fail an emissions test if there are DTCs. On the other hand, if the computer has not finished its system self-tests, the vehicle will be "not ready" for emissions testing. And the test results will be inconclusive rather than a failure.

A vehicle will fail a tailpipe test if the gas analyzer picks up excessive emissions. The test usually checks for the pollutants hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). Excessive tailpipe emissions indicate there's a problem with the car. A technician can narrow down the concern by interpreting the analyzer readings.

What should you do if your car fails?

It's pretty straightforward - if your vehicle fails an emissions test, you must fix the underlying problem. A wide range of issues can cause your car to fail. Anything from a bad gas cap to a worn engine could be to blame. So, you'll want a professional to diagnose the car and suggest an auto repair plan.

But what if your car's monitors are not ready? In most cases (but not all), the vehicle must be driven until the computer runs its tests. Sometimes, the device requires certain operating conditions before it will conduct its diagnostics, so you may want to ask a professional for advice.

What states require smog checks?

Smog inspections can be a real hassle. The good news is - the tests are only required in certain states. According to,  the states that require emissions testing include:

    • Arizona
    • California
    • Colorado
    • Connecticut
    • Delaware
    • District of Columbia
    • Georgia
    • Idaho
    • Illinois
    • Indiana
    • Louisiana


    • Maine
    • Maryland
    • Massachusetts
    • Missouri
    • Nevada
    • New Hampshire
    • New Jersey
    • New Mexico
    • New York
    • North Carolina
    • Ohio


    • Oregon
    • Pennsylvania
    • Rhode Island
    • Tennessee
    • Texas
    • Utah
    • Vermont
    • Virginia
    • Washington
    • Wisconsin

Even if your state requires emissions testing, the county you live in may not. Check with your local DMV to be sure.

What vehicles are exempt?

If you drive an older car, you may be in luck. Most states do not require emissions testing when vehicles are of a particular vintage.

The age for exemption varies by state. Arizona, for example, pardons all cars built before 1968. Meanwhile, Idaho absolves 1981 and older vehicles. Vermont is extremely lenient; it excuses all makes and models produced before 1996 from smog checks.

Alternative fuel vehicles often get off the hook as well. Many states exclude diesels, hybrids, and natural compressed gas cars from testing. Specialty vehicles like hot rods and kit cars are often excluded. And, of course, electric cars are pardoned.

A check engine light is an automatic fail on your smog check.

Tips for test day?

No matter what internet forums tell you, no magic "tune-up-in-a-bottle" will help you pass an emissions test. But you can do a few things to help ensure your car passes.

    • Make sure the check engine light is off: If you've got a vehicle with OBD, you'll want to make sure the check engine light is off. Also, if the car was built after 1996, and you have a scan tool or code reader, you should check that all (or nearly all) of the OBD monitors have run. Most states allow one or two monitors to be "not ready" during inspection. If all the required monitors have run successfully, your car will almost certainly pass its smog test.
    • Make sure your car is running well: Is your car experiencing issues like rough running, stalling, or surging? Then there's a good chance it will fail a smog check.
    • Have all your scheduled maintenance up to date: Your owner's manual outlines the manufacturer-recommended maintenance schedule. Follow it closely; it suggests services, such as spark plug replacement, that can directly impact your smog test.
    • Drive the car: Drive the car for 15 minutes or so before testing. Everything, including your catalytic converter (s), is at the proper operating temperature while your car is inspected.

What happens if you fail a second time?

If you fail your first smog test, the DMV won't register your car until you fix the problem and pass.

But what happens if you fail a second or third time? Sometimes, the DMV will issue you a temporary registration as an extension. The idea is that you will fix the car during that grace period and then get it smogged.

If you cannot get your car to pass testing, even after dumping a ton of money into repairs, the state may give you a waiver. You'll have to prove you spent a certain amount of cash attempting to fix the vehicle before issuing the waiver. Once again, this is only true in some areas, so check with your local DMV before emptying your wallet.

How much do smog checks and smog repairs cost?

The smog test is usually inexpensive - most facilities charge less than $50 for the service. But smog-related repairs are a different story. Fixing your vehicle and getting it into compliance could be anywhere from $20 (for a gas cap) to tens of thousands of dollars (for an engine). It's impossible to give an accurate estimate until the car has been professionally diagnosed.

Can you sell a car that won't pass a smog check?

Yes, you can sell a vehicle that won't pass smog. In some states, all that's required is a written agreement that states the buyer knows the car doesn't pass smog. Other states, however, require the car to be moved into Planned Operational Status before the sale. The status can be changed by contacting the DMV.

Remember that in either scenario, the buyer will need to repair the vehicle and pass smog before the DMV will issue registration.

How to find a vehicle's smog check history

Some states offer a website portal that allows you to enter a car's vehicle identification number (VIN) to retrieve the smog check history. For example, California residents can check test history through the California Bureau of Automotive Repair's website.

You can do a quick Google search or contact your local government agency to see if such information is available in your area.