Can you pass emissions with your engine light on?

Having your check engine light illuminated can cause anxiety for drivers who need to get their car to pass an emissions test. The check engine light can indicate any number of problems, some of which may not directly impact emissions. So can you pass an emissions test with the check engine light on? Here’s what you need to know.

What Causes the Check Engine Light to Come On?

The check engine light, also known as the malfunction indicator lamp (MIL), is triggered by your vehicle’s onboard diagnostics system (OBD-II) when it detects an issue with the vehicle’s emissions controls or monitors. There are thousands of diagnostic trouble codes (DTCs) that could trigger the check engine light to come on. Here are some of the most common causes:

• Oxygen sensor problems
• Catalytic converter issues
• Problems with the EVAP system
• Faulty spark plugs or wires
• Issues with the EGR valve
• Dirty mass airflow sensor
• Problems with the PCV valve or system
• Loose or faulty gas cap

As you can see, some of these directly impact the emissions system, while others only indirectly relate to emissions. So whether or not the check engine light will cause you to fail an emissions test depends on the specific reason it is illuminated.

Will You Pass Emissions With the Check Engine Light On?

In most cases, yes you can pass an emissions test with the check engine light on. Since the light could be indicating any number of issues, many may not affect the actual tailpipe emissions. However, there are a few circumstances where the check engine light will likely lead to emissions test failure:

• If the light is flashing – A flashing check engine light indicates a severe engine misfire allowing raw fuel to be dumped into the exhaust. This will cause immediate test failure.
• If the catalyst system is defective – The catalytic converter is part of the emissions control system. A faulty catalytic converter will not properly process exhaust gasses and will lead to a failed test.
• Problems with the EVAP system – The Evaporative Emissions System traps fuel vapors and prevents them from escaping into the air. Issues here can cause test failure.
• EGR system faults – The Exhaust Gas Recirculation system helps reduce emissions by recirculating exhaust back into the engine. Fault codes related to EGR will likely cause test failure.

How to Pass Emissions With the Check Engine Light On

If your check engine light is on and you need to pass an emissions test, here are some tips that may help you pass:

• Determine the trouble code(s) – Use an OBD-II scanner to pull the exact DTCs. Some codes may not cause failure.
• Reset the light – Disconnecting the battery may temporarily reset the light. It could come back on though.
• Repair known issues – Fix any problems you are aware of such as a loose gas cap.
• Add higher octane fuel – Premium fuel can help combustion issues and potentially turn off the light.
• Add a fuel cleaner – Using a fuel system cleaner can remove carbon deposits and help clean sensors.
• Replace oxygen sensors – The oxygen sensors monitor emissions performance. Swap out faulty ones if needed.
• Use an OBD-II tuner – These devices can reprogram modules to turn the light off and pass temporarily.

Keep in mind that while these tips may help you pass in some cases, the best course of action is to properly diagnose and repair the underlying issue causing the check engine light. Covering up problems temporarily should be a last resort, as continuing to drive with a malfunctioning emissions system can lead to more expensive repairs down the road.

Steps to Diagnose and Repair a Check Engine Light

Here are the general steps you should take to diagnose and repair an illuminated check engine light:

1. Retrieve the diagnostic trouble codes – Use an OBD2 scanner to pull the codes causing the light.
2. Research the specific codes – Determine the exact issues associated with the codes.
3. Inspect your vehicle – Based on the codes, inspect the related components and systems.
4. Perform any DIY repairs – Simple fixes like replacing a gas cap or spark plugs.
5. Seek professional repairs – For more complex issues like catalytic converter or sensor replacements.
6. Confirm light is off – Erase any stored codes and drive long enough for monitors to run and reset.
7. Retest emissions – Once repairs are complete, retest emissions to confirm you pass.

Following this process will ensure you not only turn the light off, but repair the root cause issue as well. Driving with the check engine light continuously illuminated can lead to further vehicle damage meaning more expensive repairs down the road.

Consult a Professional When Needed

While the check engine light may come on from minor problems, it can also indicate serious mechanical issues. Flashing dashboard warning lights in particular should never be ignored. More complex emissions and engine control systems require specialized diagnostic equipment and knowledge. If you are not comfortable troubleshooting and repairing your vehicle, consult a professional technician. Many auto parts stores offer free computer scans to read engine codes and pinpoint potential causes.

Can a Bad Catalytic Converter Cause a Failed Emissions Test?

Yes, a faulty catalytic converter is one of the most common reasons for failing an emissions test with the check engine light on. The catalytic converter is a key component of the emissions control system. It works to convert toxic exhaust gases like hydrocarbons, carbon monoxide and nitrogen oxides into less harmful emissions before they exit the tailpipe.

Here are some typical symptoms of a failing catalytic converter that can lead to emissions test failure:

• Illuminated check engine light
• Failed emissions test
• Loud exhaust noises
• Reduced performance
• Misfiring engine
• Rotten egg odor from exhaust

Replacing a bad catalytic converter will typically allow you to pass emissions testing again. Your mechanic can verify if the catalytic converter is functioning properly through diagnostics and visual inspection.

What is the Cost to Replace a Catalytic Converter?

Catalytic converter replacement costs can vary widely based on the make, model and year of vehicle. Parts costs range from $300 up to $2500 for high-end luxury and performance cars. Labor will add $100 to $250 or more to install the new converter bringing total catalytic converter replacement costs to:

• $400 to $700 for most passenger cars
• $600 to $1200 for most SUVs and trucks
• $1500 to $4000+ for high end sports cars and luxury vehicles

Aftermarket and universal converters tend to be cheaper than factory OEM catalytic converters. Convertors containing platinum, palladium and rhodium as precious metals also cost more than base steel versions.

Tips to Help Your Car Pass Emissions Testing

Beyond addressing your check engine light, here are some additional tips to help your vehicle pass emissions testing:

• Ensure the onboard diagnostics (OBD II) system is ready – Drive for several days prior and run through drive cycles if needed.
• Check tires are properly inflated – Underinflated tires increase emissions and fuel consumption.
• Change engine oil and filter – Fresh oil helps the engine run efficiently and oil filters capture carbon.
• Replace air filter – A clean air filter allows proper airflow to the engine.
• Use high quality fuel – Top tier gasoline helps minimize emissions.
• Address loose gas caps – Tighten or replace any loose, damaged or missing gas caps.
• Repair any exhaust leaks – Leaks allow raw emissions to escape before the catalytic converter.

Simple maintenance and repairs like these help ensure your vehicle is operating at peak efficiency. When components are working correctly, the emissions control system has the highest chance of keeping pollutants within acceptable limits.

Conclusion

Having your check engine light illuminated does not automatically mean you will fail an emissions test. Only certain issues directly related to the emissions systems will cause test failure. Codes should be diagnosed to determine if they will impact passing or not. In some cases, the light can be reset to try passing before more permanent repairs are made. But the best course of action is to properly diagnose and repair the issue causing the check engine light for optimum emissions control and vehicle performance.