5 Reasons Your Vehicle May Fail an Emissions Test
Did you just fail your state’s emissions test? Failing a test can be inconvenient, expensive, and frustrating. Although not required in all states, the test exists for a good reason: to minimize your vehicle’s impact on the environment.
In this article, we’ll look at the most common reasons why you might fail an emissions test, what happens if it does, and what you can do to significantly increase your chances of passing your next emissions test.
What is an Emissions Test?
Emissions testing is performed by connecting the OBD connector to a scanner at a local emissions facility. It searches for trouble codes issued by your car’s ECM/ECU and monitors the proper functioning of your catalytic converter, O2 sensors, engine, transmission, and fuel system. The scanner also takes into account daily driving habits, poor fuel economy, and high emission anomalies.
While this is standard for emissions testing today, you may also be asked to perform an “emissions analysis” during emissions testing. This involves a professional placing an exhaust gas analyzer directly into your car’s tailpipe to measure the quality and type of gases coming out of the tailpipe.
In most cases, the levels of oxygen, hydrocarbons, carbon dioxide, carbon monoxide, and nitrogen oxides are checked to see if your car meets or falls below legal limits. If you own a diesel truck or car, the concentration of soot emissions and other particles will also be checked.
Why an Emissions Test Fails
1) Faulty Fuel Injectors
If your fuel injectors are faulty or not working as the engineers intended, the mixture may be too rich. If too much fuel is not burned in the normal combustion cycle, your car will produce more carbon dioxide than normal, which becomes an emission when it leaves the exhaust. This will be clearly noticeable during an emissions test.
2) Faulty Oxygen Sensor
If the sensor that monitors oxygen levels fails, the engine will think something is wrong and will “lean” (reduce) the amount of fuel entering the cylinders. This will cause the car to go into “limp mode” and lose a lot of power, necessitating a trip to the shop for inspection. If the sensor is bad, how will the engine’s computer know if the engine is operating within the correct tolerances? This will cause it to fail emissions testing.
3) Incorrect Intake Air Values
If the test shows abnormal carbon monoxide or hydrocarbon levels, the intake system (mainly the air filter) may be clogged, dirty, or not functioning properly for some other reason. This will cause the mixture to be “lean.” This means that unburned fuel can explode in the exhaust system, releasing higher levels of greenhouse gases.
4) Loose or Leaking Gas Cap
This happens more often than you might think, but it’s easy to fix yourself. First, make sure your tank cap is tight enough. On many vehicles, you’ll hear a click, which lets you know it’s properly tightened.
If it is tight, check the seal for cracks or damage. Over time the seal will wear down, allowing fuel vapors to escape more easily. At worst, you’ll need to replace the cap, which is cheap and very easy to do yourself.
5) Check Engine Light
If the check engine light is on, whether for one of the reasons mentioned above or something else entirely, you will automatically fail an emissions test.
It is possible to reset the check engine light to stop it from coming on temporarily, but this won’t help you pass the test, as the vehicle will usually be in a “not ready” state until the various sensors have gathered data from normal driving.
With this data recorded, there is a high chance that the fault that caused the check engine light to come on in the first place will reoccur (along with the light). Multiple emissions related trouble codes can occur. These include P0141, P0410, P0411, P0430, and P0449.
What happens if I fail an emissions test?If you fail an emissions test, you will not be able to register your car in most states. This means that if you decide to drive, you will be doing so illegally until you pass an emissions test. Typically, the emissions specialist will give you a waiver so you won’t be pulled over until you return with a perfectly running car, and then you can take the vehicle to your local repair shop to have the problem fixed.