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A failed emissions test can be frustrating, especially if you’re unsure what caused it. Emissions tests are required in many areas, including Georgia, to ensure vehicles aren’t releasing excessive pollutants into the air. If your car fails an emissions test, there’s likely an issue with its fuel, exhaust, or emissions control system.
The good news? Most emissions-related failures are fixable. Below, we’ll cover the 10 most common reasons why your car might fail an emissions test, how emissions tests work in Georgia, tips for preventing failures, and how to fix these issues.
How Do Emissions Tests Work in Georgia?
In Georgia, emissions testing is required for gasoline-powered vehicles registered in Cherokee, Clayton, Cobb, Coweta, DeKalb, Douglas, Fayette, Forsyth, Fulton, Gwinnett, Henry, Paulding, and Rockdale counties. These tests help reduce harmful pollutants and improve air quality.
What Does an Emissions Inspection Test For?
An emissions inspection measures the level of pollutants your vehicle emits and ensures it meets state-mandated environmental standards. The test focuses on the following key pollutants:
- Carbon Monoxide (CO) – Produced when fuel doesn’t burn completely, high levels indicate poor combustion.
- Hydrocarbons (HC) – Unburned fuel contributing to smog and air pollution.
- Nitrogen Oxides (NOx) – A significant cause of air pollution, created at high engine temperatures.
- Evaporative Emissions – Fuel vapors escaping from the gas tank or fuel system.
If your vehicle releases excessive levels of these pollutants, it will fail the emissions test and require repairs before retesting.
What Happens During a Georgia Emissions Test?
- OBD-II Diagnostic Test: A scanner reads your vehicle’s On-Board Diagnostics (OBD-II) system to detect emissions-related faults.
- Tailpipe Emissions Test (For Older Vehicles): Older models may undergo a tailpipe test, measuring the level of pollutants emitted.
- Gas Cap Inspection: The inspector checks whether the gas cap seals properly to prevent fuel vapor leaks.
Which Vehicles Need Testing?
- Gasoline-powered cars and light-duty trucks model years 2001-2021 require testing in 2025.
- Vehicles three model years old or newer are exempt.
- Electric and diesel vehicles are exempt.
You should schedule your emissions test four to six weeks before your registration renewal deadline in case repairs are needed.
Common Reasons Why Your Car Failed Its Emissions Test
If your car produces higher-than-allowed emissions, it won’t pass the test. Here’s why this happens and how to fix it.
1. Faulty Oxygen Sensor
The oxygen sensor (O2 sensor) regulates your vehicle’s air-fuel mixture. It measures the oxygen levels in exhaust gases and relays this information to the engine control unit (ECU). If the sensor fails, the ECU can’t accurately adjust the fuel mixture, which can lead to excessive emissions, poor fuel economy, and engine inefficiency.
Signs of a Faulty Oxygen Sensor:
- Check engine light is on
- Poor fuel efficiency and sluggish acceleration
- Strong fuel smell from the exhaust
How to Fix a Faulty Oxygen Sensor
The ASE-certified technicians at McCullough NAPA Auto Care can diagnose the issue and replace the sensor if needed, ensuring your engine burns fuel efficiently and meets emissions standards for retesting.
2. Bad Catalytic Converter
The catalytic converter is responsible for breaking down harmful pollutants before they exit the exhaust. If it becomes clogged or fails, your vehicle will release excessive emissions, leading to an automatic test failure. A failing catalytic converter can also reduce fuel efficiency and overall engine performance.
Signs of a Failing Catalytic Converter
- Rotten egg or sulfur-like smell from the exhaust
- Dark smoke from the tailpipe
- Loss of power, especially during acceleration
How to Fix a Failing Catalytic Converter
The technicians at McCullough NAPA Auto Care can inspect your catalytic converter to determine if it needs cleaning or replacement.
3. Check Engine Light Is On
In Georgia, your vehicle will automatically fail its emissions test if your check engine light is illuminated. This warning light indicates potential issues with the engine, fuel system, or emissions components.
How to Fix Check Engine Light
McCullough NAPA Auto Care can perform a diagnostic scan to determine the cause for your check engine light being on and can perform the necessary repairs before your retest.
4. Too Much Fuel in the Air/Fuel Mixture
If the engine receives too much fuel and not enough air, it will produce excessive emissions. A rich fuel mixture leads to higher carbon monoxide (CO) and hydrocarbon (HC) emissions, which can cause your vehicle to fail the test.
Signs of a Rich Fuel/Air Mixture:
- Black smoke from the exhaust
- Strong fuel odor while driving
- Poor fuel economy
How to Fix a Rich Fuel/Air Mixture
The technicians at McCullough NAPA Auto Care can inspect your fuel injectors, mass airflow sensor (MAF), and fuel pressure regulator to restore proper fuel balance.
5. Loose or Faulty Gas Cap
A loose, cracked, or missing gas cap can cause fuel vapors to escape. This leads to increased emissions and can potentially trigger the check engine light.
How to Fix a Loose Gas Cap
If your gas cap is damaged, McCullough NAPA Auto Care can replace it with a proper seal to prevent vapor leaks.
6. Dirty Air Filter
A clogged air filter reduces airflow to the engine, making the engine burn more fuel and release excess pollutants.
How to Fix a Dirty Air Filter
McCullough NAPA Auto Care can replace your air filter to improve airflow, fuel efficiency, and emissions performance.
7. Old or Contaminated Motor Oil
Over time, motor oil collects carbon deposits and other combustion byproducts. If it becomes too dirty, these contaminants can enter the engine and contribute to higher emissions.
How to Fix Contaminated Motor Oil
An oil change before your emissions test can help reduce hydrocarbon emissions. McCullough NAPA Auto Care can perform this service to ensure a cleaner engine.
8. Worn-Out Spark Plugs or Ignition System Issues
Faulty spark plugs can cause engine misfires, meaning fuel isn’t burned completely. This leads to higher hydrocarbon emissions and can result in test failure.
Signs of Failing Spark Plugs
- Rough idle or engine misfires
- Slow acceleration and poor fuel economy
- Check engine light is on
How to Fix Failing Spark Plugs
A tune-up at McCullough NAPA Auto Care can replace worn spark plugs and ignition components.
9. Vacuum Leaks or Fuel System Issues
A vacuum leak allows excess air into the engine, disrupting the air-fuel ratio and increasing nitrogen oxide (NOx) emissions.
How to Fix a Vacuum Leak
A fuel system inspection at McCullough NAPA Auto Care can locate the issue, and then we can repair any vacuum leaks that we find.
10. Faulty Mass Airflow Sensor (MAF Sensor)
The MAF sensor helps regulate fuel delivery based on airflow. A failing MAF sensor can lead to incorrect fuel mixture, reducing efficiency and increasing emissions.
How to Fix a Faulty MAF Sensor
The MAF sensor may need cleaning or replacement, which McCullough NAPA Auto Care can perform before your retest.
Tips for Preventing Emissions Test Failure
- Perform Regular Maintenance – Keep up with oil changes, air filter replacements, and spark plug changes
- Address Check Engine Light Issues Promptly – Don’t ignore warning lights; get a diagnostic check early
- Drive Before Your Test – Warm up the engine by driving for 15-20 minutes before the test
- Use High-Quality Fuel – Cheap fuel can leave deposits that affect emissions
Schedule Emissions Repairs at McCullough NAPA Auto Care
If your car failed an emissions test, don’t stress. Many of the common failures can be fixed quickly with help from McCullough NAPA Auto Care. Schedule your appointment today to fix the issue and pass your emissions test!
FAQ About Why Your Car Failed Its Emissions Test
An emissions test measures the pollutants your car emits to ensure it meets environmental regulations. Many states, including Georgia, require these tests to reduce air pollution and ensure vehicles operate efficiently. If your car fails, you must repair it before renewing your registration.
Vehicles fail emissions tests due to faulty oxygen sensors, bad catalytic converters, a check engine light, rich fuel mixtures, vacuum leaks, and high hydrocarbon or carbon monoxide levels. Even simple issues like a loose gas cap or dirty air filter can cause failure.
Some minor issues—like tightening the gas cap, replacing a dirty air filter, or adding fuel system cleaner—are DIY fixes. However, problems like faulty oxygen sensors, vacuum leaks, or a failing catalytic converter require professional diagnostics and repair. McCullough NAPA Auto Care can confirm the issue and provide necessary repairs to help you pass.
- Keep up with regular maintenance, including oil changes, air filter replacements, and spark plug checks.
- Ensure the check engine light is off before testing.
- Drive at highway speeds before your test to allow the catalytic converter to function properly.
- Use high-quality fuel to prevent excessive carbon buildup in the engine.
If your vehicle fails, you will receive a Vehicle Inspection Report (VIR) explaining the issue. You must complete necessary repairs and retake the test to renew your registration. McCullough NAPA Auto Care can diagnose and repair emissions-related issues to help you pass the retest.
Failure rates vary by location and vehicle condition, but around 10-20% of cars fail emissions tests. Older vehicles, those with poor maintenance, or those with check engine lights are more likely to fail.
An emissions fault can be caused by a malfunctioning oxygen sensor, failing catalytic converter, dirty fuel injectors, vacuum leaks, faulty spark plugs, or excess carbon buildup in the engine. If your check engine light is on, it’s best to have your car inspected before your emissions test.
