Emissions tests are easy – until you don’t pass. Not only can a failed vehicle emissions test put your car out of commission due to not being able to complete the proper new or renewal registration, but a failed test could also mean that you’re contributing to the not-so-healthy air quality within your community. We know that some places in the U.S. do not require an emissions test, but perhaps you know someone who is required to pass one and could benefit from this information.

While most vehicles can easily pass emissions tests with flying colors, it’s still important to understand what can cause an emissions test failure, why it causes test failure, and what you can do about it.

1. Check Engine Light is on

The engine of your car is a remarkable machine that depends on dozens of parts and systems to work together to function as intended. When the check engine light comes on it’s a guarantee that your car will fail an emissions test. While there are many varying reasons why your check engine light is on, two of the most common reasons that directly relate to emissions include a faulty oxygen sensor and a damaged catalytic converter. We’ll talk about both for just a second.

Faulty Oxygen Sensor (02 Sensor)

When a gasoline-powered engine burns gasoline, there is oxygen present. The oxygen sensors are designed to monitor what’s happening with the fuel mixture. Over time, contaminants may accumulate on the sensor, reducing the sensor’s ability to regulate the oxygen and fuel mixture. A faulty sensor can increase the output of emissions and eventually lead to a damaged catalytic convertor. It is not uncommon to have to replace your oxygen sensors.

Damaged Catalytic Converter

Your catalytic converter has an extremely important job of taking the emissions produced by your engine and chemically converting the toxic gasses that are harmful to public health into less-toxic ones. Without a catalytic converter, your vehicle would put out far more emissions than it does.

A leaky or faulty catalytic converter will cause you to fail emissions tests almost immediately since they’ll allow many unfiltered emissions to escape. This is perhaps the most frustrating reason to fail an emissions test because replacing a catalytic converter is expensive. These tiny devices use precious metals like platinum as a part of the exhaust conversion process, so buying a new one can be extremely costly.

Of course, there are several other reasons why your check engine light may be on, many of which have nothing to do with emissions.

2. Leaking Gas Cap

This might be possibly the easiest emissions failure problem to fix. A leaking gas cap can make it impossible to recover the emissions that come from fuel evaporating in your fuel tank. This evaporation is perfectly natural but sucks the fuel out of your tank. However, thanks to your EVAP system (Evaporative Emission Control System), evaporated fuel vapors can’t escape your vehicle. Instead, you can use them to power your car when you restart it.

Your gas cap is part of the EVAP system. A faulty or leaking gas cap that isn’t containing or absorbing vapors will lead to a dry EVAP system, which usually notifies your car’s computer that something is wrong and causes you to fail emissions tests. You may not realize that your check engine light turning on may simply be because your gas cap is loose or has developed a leak. A new gas cap generally costs just a few dollars from your local auto parts store and takes just a minute or so to replace. This repair is the cheapest and easiest EVAP system repair to make on your own.

3. Overdue for An Oil Change

If you thought replacing your gas cap was easy, getting an oil change may be even easier. Your vehicle’s operating guides will tell you how often you need an oil change. Your emissions are just one of the many essential reasons you should always follow the specific oil and filter replacement recommended for your vehicle. Don’t delay your next oil change.

If ignored, the dirty oil that builds up will begin to release harmful pollutants into the air, resulting in a failed emissions test. Come into your local auto shop for an oil change if this is the case!

My Car Failed the Emissions Test – Now What?

While failing your vehicle’s emissions test can be surprising, annoying, and frustrating all in one, there is a way to get started quickly on a solution – bring your vehicle into your local Christian Brothers Automotive location. We offer digital diagnostic inspections for emissions problems and the proper maintenance services to help you retest and pass in no time at all! That’s Driving Joy.

Be sure to locate your nearest CBA shop to schedule your next vehicle emissions test along with a free courtesy inspection with one of our professional auto mechanics!

This blog was written in May 2019 and updated in August 2023 to reflect current industry standards and best practices.