Seven Reasons Why Your Vehicle Failed the Emissions Test

The older a vehicle gets the harder it is for it to pass an emissions test due to normal wear and tear. If the vehicle is maintained properly, there shouldn’t be too much of a problem, but cars, trucks, and SUVs can still fail emissions tests for any number of reasons, including age. Mark’s Auto Service runs emissions tests and can find the problem and fix it if your vehicle fails it. Here are seven reasons why it might.

Fuel Metering

If your carburetor, fuel injectors, or your vehicle’s computerized engine control is out of whack, your fuel metering may be off. This will return a failed emissions test because the vehicle will either be too rich or too lean in its air/fuel mixture. 

Oxygen Sensor

If your vehicle is pushing the wrong amount of oxygen through the exhaust, it will also fail an emissions test. The oxygen sensor measures how much oxygen is in the exhaust. If this part fails, your vehicle will fail the emissions test, too.


Maintenance can prevent this cause of an emissions test failure. Hydrocarbons in your vehicle’s exhaust can increase if you have bad ignition timing, worn spark plugs, or a damaged distributor cap. Regular tune-ups will prevent excess hydrocarbons.

Vacuum Leaks

If your MAP sensor isn’t working properly, your vehicle might begin to experience vacuum leaks. Internal combustion engines create a vacuum, which is measured by the MAP sensor. A faulty MAP sensor or leaking gaskets or vacuum lines can cause your vehicle to fail the emissions test.

Fuel Mixture

Your vehicle’s combustion engine also uses air and fuel to ignite it and keep it running. If the mixture has too much gasoline in it and not enough air, it is rich and could make your vehicle fail the emissions check. Your carburetor may be the culprit here.

Air-Injection System

We talked about hydrocarbons previously but they can also be a problem here. Your vehicle’s air-injection system controls carbon monoxide and hydrocarbon emissions. If it’s faulty, your vehicle will release too much of these dangerous gases into the air.

Evaporative Emission Control System

Also called the EVAP, this system controls the gasoline vapors from your vehicle’s fuel system. If these vapors escape into the environment, your vehicle will fail the emissions test. Your EVAP system might have leaks, which will return the F grade.

Bring your vehicle into Mark’s Auto Service for your next emissions test. We have two shops. You can call our Loves Park, Belvidere, Roscoe, and Rockford, IL, shop at 815-491-8142, and our Loves Park, IL, shop at 815-318-0200.

Photo by Welcomia from Canva Pro.

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