Why you [well, some of you] must have your car smog tested

May 1, 2017

You may not think much about that little sticker affixed to a corner of your license plate or the inside of your windshield, but it signifies an achievement for your clean-running car: passing your area's emissions test. 

It's an annual event for many car owners across the U.S. that involves a trip to a privately run garage or a sprawling government facility. Except that for just as many of us, smog testing is a totally foreign concept, thanks to the convoluted patchwork of air-quality regulations across the U.S. 

Texas inspection sticker, courtesy Flickr user stresslessauto

Texas inspection sticker, courtesy Flickr user stresslessauto

Here's what you need to know about emission testing, whether or not you're subject to it

In some—but not all—of America's most populous metropolitan areas, an annual or biennial emissions test is federally mandated. Each metropolitan area can choose how to implement testing, and the results vary considerably.

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Some require a check-up at a state-run testing station, while others ask drivers to visit licensed garages less often.

Though the end goal for this crazy quilt of legislation is the same everywhere—cleaner air via lower emissions of carbon monoxide, carbon dioxide, hydrocarbons, and oxides of nitrogen—its implementation is far from consistent. 

It all dates back to amendments to the Clean Air Act passed in 1970, which gave the federal government the power to regulate what could legally be emitted from a new vehicle's tailpipe. Previously, federal motor vehicle regulation was limited primarily to safety standards. Automakers raised red flags over their concerns about the cost and challenges of cleaning up emissions, but ultimately the Clean Air Act has worked well to do exactly what its name implies. 

MORE: What happens during a smog check?

In 1990, the EPA created National Ambient Air Quality Standards (NAAQS) that required the nation's dirtiest cities to clean up what their citizens breathed. Idling cars are among the biggest polluters, so the EPA requires states to measure what comes out of tailpipes at idle.

Each state enforces NAAQS somewhat individually, though the EPA has ultimate approval of the testing techniques required. Some states require testing for all cars. Others exempt vehicles 20 or 30 years old, or those with certain license-plate types.

Smog test, California, courtesy Flickr user viewfrom52

Smog test, California, courtesy Flickr user viewfrom52

Most don't have statewide testing, instead reserving it to metro areas with lower air qualities. For instance, an Indiana vehicle registered near Chicago is required to be tested every other year while one registered in Indianapolis never has to be tested. The EPA determined that the air quality in the Chicago area is poor enough to require biennial testing, but that the air in Indianapolis is clean enough as is.

How each state handles emissions testing costs also varies wildly. States like Illinois and Wisconsin don't charge fees for an emissions test. Others, like New York and Texas, bundle emissions testing into an annual safety inspection.

Smog over Los Angeles, courtesy Flickr user steven-buss

Smog over Los Angeles, courtesy Flickr user steven-buss

The Golden State challenge

It's a different story in California, where the California Air Resource Board (CARB) essentially operates as a localized EPA with a far heavier hand. More than 10 percent of all new cars sold in the U.S. are bought by Californians, so CARB's influence is felt nationwide, if not globally.

As a result, automakers began developing more restrictive emissions control systems for new cars sold in California. Versions of some vehicles—specific engines, typically—are occasionally not certified for sale in the Golden State, even though they are offered just over the border in Arizona or Nevada. This practice has become less common recently, however. 

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Several Northeastern states have adopted California emissions standards, although most cars sold today don't differ significantly from state-to-state. 

Unlike most other states, California requires emissions testing for every car built in 1975 or later. (The state also provides some small incentives for low-income drivers to fix up older cars.) CARB requires old vehicles to have fully functioning emissions systems that operate the same today as they did when new.

Rapidscreen mobile emissions testing in Colorado, courtesy Flickr user elisfanclub

Rapidscreen mobile emissions testing in Colorado, courtesy Flickr user elisfanclub

The testing process

Like the selection of places where emissions testing is required, the actual test itself can vary greatly. On some older vehicles, many states hook a sensor into a vehicle's tail pipe to measure what is actually emitted. 

On newer cars, the federally mandated On-Board Diagnostic system fitted at the factory (known as an OBD, or OBD-II system on the latest vehicles) lets testing facilities tap directly into a vehicle's computer system to read what its myriad sensors have to say. Generally, this kind of emissions testing is performed on a dynamometer that uses rollers to reproduce highway driving while stationary. 

Finally, some states will set up roadside emissions-testing facilities that monitor vehicles as they drive past sensors. These roadside emissions tests take a photo of the car's license plate while they're measuring exhaust emissions, and the state will alert owners of emission violations via a postcard sent in the mail. 


Before you get your next emissions test, it may be worth consulting with your state's motor vehicle enforcement agency to see if you qualify for an exemption.

Some states may exempt vehicles from emission tests for one of the following circumstances:

  • A car didn't pass, but the owner can prove they've already spent more than a certain amount trying to fix its emissions system. 
  • Cars are older than a certain age; as recent as the 1996 model year in some places.
  • Gross vehicle weight ratings exceed 9,000 pounds.
  • Cars are registered with certain collector or antique license plates.

Additionally, not all states will test diesel-fueled vehicles, natural-gas vehicles, or motorcycles. 

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