Why Your Car Might Emission Test Fail and How to Fix It

Discover why emission test fails happen and how to address them in our latest blog. Learn essential tips for passing your emission test stress-free!

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Imagine the frustration of taking your car in for an emission test fail, only to have it fail. You’re left wondering what went wrong, and what you can do to fix it. The good news is that understanding the common causes, symptoms, and preventive measures can help you avoid this situation altogether. In this blog post, we’ll explore the reasons behind emission test fail occurrences and provide practical tips to ensure your vehicle passes with flying colors.

Key Takeaways

  • Common causes of emissions test failure include faulty oxygen sensors, worn spark plugs, and problems with the catalytic converter.

  • Simple fixes to pass an emissions test include tightening the gas cap, replacing the air filter, getting an oil change and investing in diagnostic tests.

  • Preventive maintenance such as regular inspections and tune-ups can help ensure successful passing of emissions tests.

Common Causes of Emission Test Failure

A car failing an emissions test

Emissions test failures can often be traced back to a few common issues involving the vehicle’s exhaust system. These issues include:

  • Faulty oxygen sensors

  • Worn spark plugs

  • Problems with the catalytic converter

  • Malfunctioning mass airflow or oxygen sensor

  • Clogs, leaks, or an imbalance in the air-fuel mixture

All of these can contribute to your vehicle’s inability to pass the emissions test.

Other potential culprits that could lead to a failed test, specifically emissions test failure, include:

  • Outdated motor oil

  • A defective EVAP system

  • An overly rich-running system

  • A clogged air filter

We will further explore each of these common causes, explaining their impact on your vehicle’s exhaust and how they could lead to emissions test failure in the exhaust system.

Faulty Oxygen Sensor

A malfunctioning oxygen sensor can wreak havoc on your vehicle’s air-fuel mixture, resulting in a failed emissions test. The oxygen sensor measures the amount of unburned oxygen in the exhaust, which is essential for emissions testing. An incorrect air-fuel mixture can be caused by a faulty oxygen sensor or a malfunctioning mass airflow sensor, leading to higher emissions and potential test failure.

Often, the recommended solution for emission test failure due to a faulty oxygen sensor is its replacement. This simple fix can save you from more expensive repairs down the road, such as catalytic converter replacement, and help ensure your vehicle passes the emissions test.

Worn Spark Plugs

Worn spark plugs can lead to:

  • Unburned fuel

  • Higher emissions

  • Reduced fuel economy

  • Lack of acceleration

  • Hard starts

  • Engine misfires

  • Rough idling

Ignoring these signs can cause your vehicle failed emissions test due to increased emissions from unburned fuel.

Inspecting and replacing worn spark plugs as needed is key to avoiding this issue. Regular maintenance and replacement of spark plugs can help your vehicle run more efficiently, ultimately reducing emissions and increasing the likelihood of passing the emissions test.

Catalytic Converter Problems

Catalytic converter issues can be another major cause of emissions test failure, as they can lead to increased carbon monoxide levels. The purpose of a catalytic converter is to transform carbon monoxide into carbon dioxide, helping to reduce harmful emissions. The catalytic converter is a critical component for passing emissions tests. If it is damaged, this can lead to the vehicle failing the test..

Typical indications of a failing catalytic converter include:

  • Slow powertrain performance

  • Reduced acceleration

  • A smell of rotten eggs or sulfur from the exhaust

  • Dark exhaust smoke

  • The Check Engine Light illuminated

  • Sluggish acceleration

  • Decreased fuel efficiency

  • Difficulty starting the car

  • Poor engine performance and acceleration

  • Heat emanating from beneath the vehicle

Repairing or replacing the catalytic converter as needed to address these signs is critical to preventing more serious issues and ensuring a successful emissions test.

Emission Test Failure Symptoms

A car with a check engine light warning

Now that we’ve discussed the common causes of emissions test failure, let’s explore some of the symptoms that may indicate a potential problem. These symptoms include decreased fuel efficiency, increased engine noise, and check engine light warnings. Recognizing and addressing these symptoms can help you prevent test failure and ensure your vehicle is running at its best.

By understanding the relationship between these symptoms and their underlying causes, you can take action to resolve any issues before they lead to a failed emissions test. We will further discuss each symptom in detail and suggest solutions to help you pass the emissions test.

Decreased Fuel Efficiency

A decrease in fuel efficiency could potentially be an indication of emission-related issues, such as an oxygen sensor malfunction or a defective catalytic converter. Common causes of decreased fuel efficiency include improper air flow, faulty injectors, bad oxygen sensors, misfiring spark plugs, and the aging of the vehicle. A malfunctioning oxygen sensor or catalytic converter can also lead to a decrease in fuel economy, increased exhaust emissions, and reduced engine performance.

Addressing these issues by replacing the oxygen sensor or catalytic converter, as well as ensuring proper maintenance of other components, can help improve fuel efficiency and reduce the risk of emission test failure. Maintaining optimal fuel efficiency and emissions performance requires regular inspections and maintenance of these components.

Increased Engine Noise

Increased engine noise may suggest exhaust system issues, which could lead to emission test failure. The usual causes of increased engine noise associated with emission test failure are typically:

  • A faulty exhaust system

  • A faulty oxygen sensor

  • A faulty fuel injection unit or carburetor

  • Clogs or leaks in the emission system

  • Old or defective spark plugs

Leaks or cracks in the exhaust system can cause exhaust gases to escape before reaching the muffler, resulting in a louder sound. In addition, blockages in the exhaust system can cause a chugging noise, thus further amplifying the engine noise.

Identifying and addressing these issues will ensure proper functioning of your vehicle’s exhaust system, thereby reducing the risk of emissions test failure. Regular inspections and repairs of the vehicle’s exhaust system components can help to maintain optimal engine performance and emissions levels.

Check Engine Light Warnings

Check engine light warnings can signal emission-related issues that need to be addressed before testing. The most frequent causes of the check engine light illuminating in relation to emissions are:

  • A loose fuel cap

  • Oxygen sensor failure

  • Catalytic converter failure

  • Spark plug/ignition coil issues

  • Wiring issues

The On-board diagnostic system (OBD) is designed to detect issues in the engine or emission control system and notify drivers by illuminating the check engine light on the dashboard. If the OBD system detects an emissions-related issue, the vehicle may fail an emission test.

Promptly addressing check engine light warnings is crucial to prevent further damage to emission control components and ensure your vehicle passes the emissions test. Regular inspections and maintenance can help to resolve any underlying issues and keep your vehicle running efficiently.

Simple Fixes to Pass the Emissions Test

Sometimes, the solution to passing the emissions test can be as simple as tightening the gas cap, replacing the air filter, or getting an oil change. These simple fixes can help to improve your vehicle’s emissions performance and increase the likelihood of passing the test. By addressing these minor issues, you can avoid more costly repairs and ensure your vehicle is in compliance with emissions regulations.

We’ll further discuss these simple fixes, detailing how they can contribute to passing the emissions test and maintaining your vehicle’s optimal performance.

Tightening the Gas Cap

A loose or leaking gas cap can cause excess fumes to escape, leading to a failed emissions test. Indications of a leaking gas cap may include:

  • A gas cap that is not securely fastened or does not tighten securely

  • A pungent fuel odor

  • A reduction in fuel efficiency

  • The activation of the “Check Engine” light

A loose gas cap can have a detrimental effect on a vehicle’s emissions, as fuel vapors are able to escape and cause errors in the emissions system, thus resulting in increased emissions.

To resolve this issue, simply tighten or replace the gas cap as needed. Ensuring a secure gas cap can help to reduce emissions and increase the chances of passing the emissions test.

Replacing the Air Filter

A dirty air filter can lead to increased emissions and test failure. The air filter is responsible for filtering out contaminants from the air before it enters the engine, ensuring optimal combustion and reducing emissions. A clogged air filter can restrict airflow, causing the engine to work harder and produce higher emissions.

Replacing the air filter is a simple and cost-effective solution to improve emissions test results. It is recommended to change the air filter annually or when the vehicle has driven 12,000 miles, whichever occurs first. Regular air filter replacements can help to maintain optimal engine performance and reduce the risk of emissions test failure.

Getting an Oil Change

Old motor oil can increase hydrocarbon emissions and lead to test failure. Regular oil changes are essential for reducing car emissions, as clean oil helps to:

  • Reduce pollutants

  • Provide a smoother driving experience

  • Keep engine components lubricated

  • Improve performance and fuel economy

  • Ultimately result in lower emissions

On the contrary, dirty oil can increase emissions and lead to air pollution.

Getting an oil change before undergoing an emissions test can help to improve test results and ensure your vehicle is running efficiently. Using the recommended motor oil and adhering to the manufacturer’s suggested oil change schedule can help to optimize vehicle emissions and prevent test failure.

Diagnostic Tests and Professional Help

A car with a diagnostic test being performed

While simple fixes can often resolve emissions test issues, sometimes more advanced diagnostic tests and professional help are necessary to identify and resolve the problem. Diagnostic tests can accurately pinpoint the cause of emission test failure, allowing for appropriate repairs to be made. Seeking professional assistance can provide the expertise and tools necessary to guarantee effective repairs and increase the likelihood of passing the emissions test.

We’ll further delve into the benefits of diagnostic tests and the importance of professional assistance in resolving emission test issues.

Diagnostic Test Benefits

Diagnostic tests can precisely identify the source of emission test failure, thereby enabling specific repairs to be made. These tests are designed to analyze the various components of the vehicle’s emissions system and detect any abnormalities or malfunctions. By measuring the levels of pollutants in the exhaust gases and analyzing the performance of emission control devices, diagnostic tests can accurately pinpoint specific emission issues, making them a valuable tool in addressing emissions test failure.

Regular diagnostic tests can help to:

  • Identify and resolve minor issues before they escalate into more significant problems

  • Prevent emissions test failure

  • Maintain optimal vehicle performance and emissions levels

  • Ensure a successful emissions test outcome.

Early detection and timely repairs are key in achieving these benefits.

Seeking Professional Assistance

Professional assistance can guarantee proper repairs and increase the chances of passing the emissions test. A qualified and trained automotive service technician, equipped with the necessary diagnostic repair tools, and knowledgeable about emissions systems and regulations is essential for effectively addressing emission test failures. Professional assistance can provide a range of services, including:

  • Emissions failure repairs

  • Exhaust services

  • Emissions repair services

  • Vehicle inspection and maintenance programs

By engaging the services of a professional, you can trust that the necessary repairs will be made correctly and efficiently, ultimately increasing the chances of passing the emissions test and maintaining your vehicle’s optimal performance.

Preventive Maintenance for Emissions Test Success

A car with a regular inspection being done

Preventive maintenance is crucial for ensuring emissions test success. Regular inspections, tune-ups, addressing warning signs, and cleaning the catalytic converter can all help to maintain your vehicle’s efficiency and reduce the risk of test failure. By staying proactive with your vehicle’s maintenance, you can avoid costly repairs and ensure that your vehicle remains in compliance with emissions regulations.

We’ll delve further into each aspect of preventive maintenance, detailing how each can aid in passing the emissions test and maintaining your vehicle’s optimal performance.

Regular Inspections

Regular inspections can identify potential emission-related issues before they cause test failure. By adhering to state regulations and conducting regular inspections, you can ensure your vehicle is in optimal condition and address any issues promptly. Inspections should focus on critical elements such as:

  • The exhaust system

  • The catalytic converter

  • The oxygen sensors

  • The fuel system

  • Emission control devices

  • On-Board Diagnostics (OBD) System

Regular inspections and addressing potential issues can help maintain your vehicle’s efficiency and emissions performance, thus increasing the likelihood of emissions test success.

Tune-ups and Maintenance

Tune-ups and maintenance can keep the vehicle running efficiently and reduce the risk of emission test failure. A tune-up typically involves examining the engine for components that require cleaning, repairing, or replacing, such as spark plugs, spark plug wires, and engine air filter. Regular tune-ups can assist in boosting fuel efficiency, reducing emissions, and guaranteeing that your vehicle passes emissions tests.

Adhering to the manufacturer’s maintenance schedule and performing regular tune-ups can assure optimal vehicle performance and lower the risk of emissions test failure.

Addressing Warning Signs

Addressing warning signs, such as check engine light warnings, can prevent emission test failure. Ignoring these warnings can lead to more costly repairs and a higher likelihood of test failure. The check engine light can indicate issues with:

  • System sensors

  • Engine control modules

  • Catalytic converter failure

  • Oxygen sensor malfunction

  • Other engine problems related to emissions.

Taking prompt action to address warning signs can maintain your vehicle’s emissions performance and increase the chances of passing the emissions test.

Cleaning the catalytic converter with LIQUI MOLY Catalytic Converter Cleaner

A close-up image of a catalytic converter being cleaned with LIQUI MOLY Catalytic Converter Cleaner after an emission test fail.

Cleaning the catalytic converter with LIQUI MOLY Catalytic Converter Cleaner can help maintain its efficiency and improve emission test results. The cleaner works by safeguarding the catalytic converter from becoming clogged with soot, cleaning intake valves and the combustion chamber, and improving the combustion of fuel. It is recommended to use the cleaner every 6000km or as specified on the product label.

LIQUI MOLY Catalytic Converter Cleaner can enhance the catalytic converter’s effectiveness, reduce emissions, and improve chances of passing the emissions test. However, following the manufacturer’s instructions and safety precautions is key to avoiding potential risks from improper use.


In conclusion, understanding the common causes, symptoms, and preventive measures of emissions test failure is essential to ensure your vehicle passes the test. By maintaining your vehicle with regular inspections, tune-ups, addressing warning signs, and cleaning the catalytic converter, you can keep your vehicle running efficiently and reduce the risk of test failure. Invest in your vehicle’s maintenance to enjoy a smoother, cleaner ride, and pass your emissions test with confidence.

Frequently Asked Questions

How many times can you take the emissions test in Illinois?

In Illinois, vehicles must be tested every two years, and newly purchased used vehicles must be tested within 30 days.

What happens if you fail emissions test Illinois?

If you fail the emissions test in Illinois, you will have to pay for repairs to ensure your car passes. After that, you can renew your registration.

What causes co emissions failure?

CO emissions failures are generally caused by issues with the carburetor or fuel delivery system, resulting from too much fuel and not enough air. Spark plugs do not fix CO failures.

What are the most common causes of emission test failure?

The most common causes of emission test failure are faulty oxygen sensors, worn spark plugs, and catalytic converter issues, making it important to address these issues before testing.

What are the symptoms of a failing catalytic converter?

Symptoms of a failing catalytic converter include reduced acceleration, slower performance, sulfur-like smell from the exhaust, dark smoke from the exhaust, and the Check Engine Light illuminated.

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