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Emission Test Requirements
Most inspection requirements are linked to your vehicle’s registration notice, so if your vehicle requires emissions testing, the renewal notice will indicate the requirements. If you fail your emissions test, you probably won’t be allowed to register your vehicle.
Over 30 states require some form of emission testing. Some states call their testing smog inspections. A few states incorporate the emissions test into their yearly safety inspection. Other states only require cars registered in certain metropolitan areas to get the tests.
Sometimes, only specific older makes must have emission testing. New vehicles often do not need an emissions test for the first five years of ownership.
What Emissions Tests Look For
There are three emissions testing standards. The type of testing depends on your state, the year of your car, and the engine type.
- Tailpipe emissions testing. Depending on the model year and the testing location, a vehicle is driven on a dynamometer at either varying speeds or at a steady speed while a device measures the emissions at the tailpipe.
- On-board diagnostics (OBD). This test is the easiest and is commonly performed on cars newer than 1996. A technician hooks a diagnostic device to the car’s OBD port, and the engine operating data is accessed and reported.
- Smoke density. For diesel engines, an instrument measures the opacity of the exhaust gasses, usually while on a dynamometer to simulate driving conditions.
Emissions testing for gasoline engines commonly checks for three gasses at the tailpipe:
- Hydrocarbon (HC) – The presence of hydrocarbons is an indication that the engine does not fully burn raw fuel.
- Carbon monoxide (CO) – CO emissions typically indicate incomplete combustion at the engine.
- Nitrogen oxides (NOx) – These gases, also called oxides of nitrogen, come from engines with excessively high internal combustion temperatures.
All three gasses contribute to air pollution. Emissions testing aims to lower these gasses by requiring the owner to maintain proper engine operations, especially the emissions system.
Many states that test also evaluate the amount of sulfur dioxide (SO2) and particulate matter (PM) coming out of the tailpipe. Sulfur dioxide affects breathing for people with asthma. Particulate pollution (that is relative small) is commonly called soot. These pollutants are tested for direct impact on human health rather than the environment.
Get Your Car Ready for Testing
If your check engine light is on, you will certainly fail emissions testing. This light indicates one or more sensors reporting an error. Until the error is fixed, most states will fail the car. So fix that issue before attempting an emissions test.
Some owners might attempt to clear the check engine light and related pesky codes by disconnecting the battery or using a code reader to temporarily remove those codes. But the OBD system might not indicate the necessary “ready state” to allow a valid emission test until the car has driven enough cycles to regenerate that deleted check engine code.
If the battery was disconnected, a ready state will typically occur after a car was started five to 10 times and driven at least 50 miles.
The Testing Process
First, locate a test site approved by your state. In some states, local automotive shops can conduct emissions testing. In others, specific sites contracted by the state do these tests. Check with the site to ensure they can perform the specific test your state requires for your vehicle.
Once you arrive at the site, a technician will direct you to a testing bay. You then exit your car while the technician runs the vehicle through the testing process.
Technicians might also do a cursory check of the engine bay to make sure the emission equipment installed by the manufacturer is still in place. They might also test the gas cap to make sure it is making a good seal.
The test usually takes about five minutes. But if you wait until the end of the month, you might experience longer lines with all the other owners who waited until the last days of the month to complete the test and pay registration fees.
When the test is complete and the car is moved off the testing area, you get a printout showing whether you passed or not. If your car failed the test, the printout should indicate which tests were performed and the specific readings. The printout should help you figure out how to correct any problems.
Pass or fail, emissions tests cost less than $25 in most states. If a shop is conducting the test, their fee might vary depending on the local market. Check online for discount coupons, which are often offered by local shops.
A smog check might raise your anxiety level. In most cases, there’s no need to worry. A properly functioning car should breeze through the test.
