Common Reasons for Failed Emissions Test

Many states, cities and municiplaities throughout the United Stated have clear air laws requiring drivers of certain vehicles to pass an emissions test, usually annually. If your vehicle fails this test, you won't be able to renew your tag until the problem is fixed. The fundamental purpose of an emissions test is to ensure the vehicle isn't releasing a significant amount of harmful gas into the environment. So, what are some of the most common reasons why vehicles fail this test?

Faulty Gas Cap

If your vehicle fails the emissions test, cross your fingers and hope the problem is related to a bad gas cap. This is the cheapest and easiest problem to fix. When the gas cap loses its ability to create an air tight seal, some of the fumes will escape; thus, increasing the risk of a failed emissions test. You can buy a new gas cap for $5-$10 bucks, however, making this a quick, easy and inexpensive fix.

Bad Oxygen Sensor

Another common reason for a failed emissions test is a bad oxygen (O2) sensor. The O2 sensor is responsible for measuring the amount of oxygen in the gas. When it goes bad, it can lead to problems like overheating, higher emissions, and loss of power.

Rich Fuel Mixture

What is a rich fuel mixture and how it can lead to a failed emissions test? This occurs when high levels of carbon monoxide, which is often the result of a weak ignition. If your vehicle burns too much gas, the fumes which are created as a byproduct will appear during the emissions test; thus, causing your vehicle to fail the test. 

Cracked Exhaust Manifold

A cracked or otherwise damaged exhaust manifold can also lead to a failed emissions test. The exhaust manifold is responsible for collecting gases from multiple cylinders and diverting them into a single pipe, which then leads to the exhaust. If the exhaust manifold is cracked, it may leak some of these gases -- and this could prevent your vehicle from passing an emissions test.

Faulty Air Injection System

A fifth reason why vehicles fail their emissions test is because they have a faulty air injection system. The air injection system is intended to reduce emissions (hydrocarbons and carbon monoxide) by forcing fresh air into the exhaust. When it goes bad, it's unable too flush these emissions from the engine, causing the vehicle to produce an excessive amount.