Each state has different procedures for vehicle emissions testing, but common procedures include checking the fuel tank filler restrictor, ensuring that the gas cap seals tight, looking for the catalytic converter under the vehicle and looking at the instrument panel to see if the "Check Engine" light is illuminated. Most states use emissions testing to check for unburned hydrocarbons and carbon monoxide, and other states additionally check for carbon dioxide for diagnostic reasons.
California has additional emissions testing requirements. These include looking under the vehicle's hood to ensure that the engine has all required emission control parts correctly installed and verifying that any installed aftermarket parts are emissions certified. Replaced engines must meet emission requirements for the vehicle's model year.
Minor problems may cause a vehicle to fail its emissions test. If the driver travels only a short distance to the testing facility, the vehicle's engine may not reach normal operating temperature that may cause it to fail the emissions test. Dirty spark plugs, air filter and oil may also cause the vehicle to fail. A single misfiring spark plug may increase hydrocarbon emissions up to 10 times the standard amount. Paying for emissions repairs is the vehicle owner's responsibility once the vehicle's warranty expires.