Will I pass emissions if I clear my check engine light?

Quick Answer

Clearing the check engine light prior to an emissions test will not guarantee that you pass emissions. The emissions monitors that test various engine and emissions components may not have completed their drive cycles and tests after the light is cleared. However, clearing the light can sometimes help in certain situations.

What Causes the Check Engine Light to Come On?

The check engine light (also known as the malfunction indicator lamp or MIL) illuminates on the dashboard when the vehicle’s on-board diagnostics (OBD-II) system detects a problem with the engine, emissions system, transmission, or other monitored components.

Some common causes for the check engine light include:

• Loose or faulty gas cap
• Faulty oxygen sensor
• Bad catalytic converter
• Worn spark plugs and wires
• Faulty mass airflow sensor
• Loose positive and negative battery cables
• Failed evaporative emissions system components
• Faulty EGR valve

The OBD-II system has various monitors that test components to ensure they are operating properly and within specifications. Monitors include tests for the fuel system, oxygen sensors, EGR system, catalytic converter, and other emissions controls.

When the OBD-II system detects a problem that impacts emissions, it turns on the check engine light and stores a diagnostic trouble code (DTC) to indicate which component or system is malfunctioning. A diagnostic scan tool is required to read the codes.

How Do Emissions Tests Work?

Emissions tests, also known as smog checks, are performed on vehicles to ensure they meet emissions standards and do not pollute the air. Emissions tests are required by law in many states and jurisdictions on a periodic basis for all vehicles or at change of ownership.

There are two main types of emissions tests:

OBD-II Plug-In Emissions Test

This is the most common test used today. A scanner is plugged into the vehicle’s OBD-II diagnostic port, usually located under the dash. The scanner connects to the OBD computer and checks to see that all emissions monitors have completed their tests and the vehicle is in compliance with emissions standards.

Tailpipe Emissions Test

This older method measures the actual tailpipe emissions by placing a probe in the exhaust pipe while the engine is running. Emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) are measured and compared to state or federal standards.

Do Monitors Need to Complete Their Drive Cycles?

For newer vehicles that use the OBD-II plug-in test, typically the emissions monitors need to complete their drive cycles and run tests before the test. Some key monitors that need to be set as “Ready” or “Complete” prior to emissions testing include:

• Fuel system monitor
• Misfire monitor
• Comprehensive component monitor
• EGR system monitor
• Oxygen sensor monitor
• Catalyst monitor
• Evaporative system monitor

If too many monitors show “Not Ready” or “Incomplete” when scanned prior to an emissions test, the vehicle may fail the inspection.

Monitors run tests and complete drive cycles as the vehicle is driven normally, often needing a certain set of conditions, mileage, and drive time to reset.

How Does Clearing the Check Engine Light Affect Monitors?

When the check engine light is cleared, either by disconnecting the battery or using a diagnostic scan tool, most emissions monitors are reset to a “Not Ready” status. This essentially erases their memory and starts them over from scratch.

The monitors then need to complete an entire drive cycle over again to run tests and check for faults before reporting a Ready or Complete status. This can take quite a bit of driving, often a week or more for some monitors.

So although clearing the MIL will turn off the warning light, it also resets the monitors that emissions tests check as part of the OBD-II plug-in test.

Will Clearing the Codes Guarantee Passing Emissions?

No, clearing diagnostic trouble codes and turning off the check engine light does not guarantee passing an emissions test. Since it resets the monitors to “Not Ready”, it can actually cause an immediate failure.

The emissions monitors may take many days and miles of driving to run their diagnostic tests and complete their drive cycles. If the root cause that originally triggered the check engine light has not been repaired, the light will simply illuminate again once the monitor detects the fault.

However, clearing codes can help in certain situations when attempting to pass emissions.

When Can Clearing the Light Before Emissions Testing Help?

Here are some examples where clearing the check engine light before emissions testing may improve your chances of passing:

#1 Loose Gas Cap Codes

A loose or faulty gas cap is one of the most common causes for check engine lights. It triggers an evaporative system leak detection monitor fault.

If the gas cap is subsequently tightened or replaced, clearing codes to turn off the MIL allows the evaporative monitor to reset and complete its drive cycle during normal driving. As long as no other problems are present, emissions can then pass.

#2 Faulty Components Replaced

If you had a faulty oxygen sensor, spark plugs, catalytic converter or other component that triggered the check engine light, and you repaired and replaced the faulty component, clearing the codes may allow monitors to run and test properly to complete their drive cycles without detecting faults.

#3 Drive Cycles Completed

If you recently had a check engine light, but have put significant driving miles on the vehicle after the light illuminated, the monitors may have already completed their diagnostic testing. Clearing the light turns it off without resetting the monitors. An emissions test quickly after clearing may pass in this situation.

Steps to Clearing Check Engine Lights

Here is the general process if you want to attempt clearing the check engine light before emissions testing:

1. Repair any known faulty components that caused the light to turn on
2. Drive the vehicle until monitors have a chance to complete (50-200 miles)
3. Recheck codes and monitor status with a diagnostic scanner
4. Clear any remaining trouble codes
5. Complete drive cycles and follow monitor status
6. Test emissions soon after monitors are in a “Ready” status

Risks of Clearing Codes Before Emissions Testing

Although it may help in some cases, there are risks to erasing the check engine light before emissions inspections:

• Monitors may not complete in time and the vehicle fails with “Not Ready” monitors
• The root problem has not been fixed and causes the light to re-illuminate
• Clearing light prevents diagnosing and repairing the true issue
• In some regions, clearing codes just before testing is against the law

Consult your vehicle repair manual and emissions regulations for your area when considering clearing the check engine light before an emissions test.

Conclusion

Clearing check engine lights before emissions testing does not guarantee a pass. Recently cleared vehicles often fail from monitors not being in a ready state. However, if repairs have been made and significant driving has occurred after a fault, clearing codes can reset monitors and turn off the MIL to assist in passing. Knowledge of monitor drive cycles and local regulations are important when considering resetting OBD-II systems prior to emissions inspections.

Situation	Will Clearing Codes Help Pass Emissions?
Loose gas cap recently fixed	Yes, allows evaporative monitor to reset
Faulty part replaced over 500 miles ago	Possibly, depends on monitor status
Check engine light on with low fuel level	No, fuel level sensor fault detected
Light cleared day before testing	No, monitors likely not ready
Car driven two weeks since light on	Maybe, monitors may be complete

References

• EPA Emissions Monitors Fact Sheet – https://www.epa.gov/obd/emission-monitors-fact-sheet
• Your Mechanic – How to Pass Emissions With Codes – https://www.yourmechanic.com/article/how-to-pass-an-emissions-test-with-a-check-engine-light-on-by-cheryl-knight
• AA1Car – Emissions Monitor Drive Cycles – https://www.aa1car.com/library/monitor_drive_cycles.htm
• CarMD – Emissions Drive Cycle Info – https://www.carmd.com/wp/emissions-drive-cycle-information/