5 Reasons for Failing an Emissions Test (and Help Identify Problems)

Emissions are the exhaust gases that emit from the engine of a vehicle. In many states, vehicles are required to undergo emissions tests every 6 months to make sure they aren’t producing too much carbon monoxide. They also look for dangerous pollutants that might be in the gasses as well.

If it is found that your engine does not pass the emissions test, you won’t be allowed to register your vehicle until you get the problem repaired. This is the problem that is causing your car to produce high emissions.

See Also: 7 Signs You Have a Hole in Your Muffler

Top 5 Causes of a Failed Emissions Test

There are many reasons as to why you have high enough car emissions to fail an emissions test. Most of the causes won’t cost too much to fix. Since you can’t legally drive your vehicle in many jurisdictions without passing the emissions test, getting these problems repaired is essential.

Below are 5 common reasons for high emissions that result in test failure:

1) Fuel Injection Problem

fuel injectors

The engine control unit is what manages the fuel injector, which is responsible for injecting fuel into the internal combustion chamber.

If either the engine control unit or the fuel injector was to malfunction and cause an imbalance of the air and fuel ratio, it would create more emissions that would get released.

2) Bad Mass Air Flow Sensor

bad maf sensor

The engine control unit depends on the mass air flow sensor to calculate the mass rate of the air that is flowing into the internal combustion engine. Once the engine control unit has this information, it will know the proper amount of fuel mass to send into the engine.

If this mass air flow sensor were to malfunction, then it would likely cause the fuel to be burned too much. This is called a rich fuel mixture, which creates excessive amounts of carbon monoxide.

3) Bad EVAP System

loose gas cap

The EVAP system is the Evaporative Emission Control System which most modern cars have. Their job is to stop gas fumes from being released into the atmosphere. Some of the parts of this system include the vents, vacuum hoses, and gas cap.

If the cap were to become loose or the hoses were to break, then these fumes could leak through and cause a big problem.

4) Bad MAP Sensor

bad MAP sensor

The Manifold Absolute Pressure Sensor, also called the MAP sensor, provides the engine control unit with data about the manifold pressure.

If there were to be leaks in any of the vacuum lines or gaskets, then it is probably due to the MAP sensor malfunctioning. This would create a high emissions problem from the engine.

5) Bad Oxygen Sensor

bad oxygen sensor

Vehicles have what is called an oxygen sensor. The purpose of this sensor is to examine the exhaust gas and determine how much oxygen is in it.

If this sensor were to become defective, it could lead to an increase in the toxicity of the emissions or cause the engine to overheat.

Related: Will a Muffler Delete Pass Emissions?

How to Detect These Problems

Sometimes the problems listed above will cause an impairment to your driving that is easily noticeable. For example, if the oxygen sensor were to go bad, then your acceleration power would likely be reduced as you step on the gas pedal.

Otherwise, you won’t really know if you’re producing high emissions until you take your vehicle for its biannual emissions test. If you live in a state like Florida which does not require emissions tests, then you will never know until you take your vehicle in for a checkup or a diagnostic test.

That is why it is important to know the main causes of high emissions, so you can perform the diagnostic test yourself and be prepared to pass the emissions test beforehand.


4 thoughts on “5 Reasons for Failing an Emissions Test (and Help Identify Problems)”

  1. I have nissan tida 2007 model 1.6.car have fume emission. High co

    cleaned the fual injection. And spark plug.Have little bit smoke nw. Wat is the reason for this?
    Hw can prevent?


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