Will You Fail Emissions with the Check Engine Light On?
Can You Pass an Emissions Test with the Check Engine Light On?
No, in most states, you will fail the emissions test if the check engine light (CEL) is on. The emissions system monitors pollutants, and an active CEL usually indicates an issue that affects emissions performance.
Why Does the Check Engine Light Cause Emissions Failure?
- OBD-II System Reads a Stored Error Code – Emissions tests use an On-Board Diagnostics (OBD-II) scan, and if an active trouble code is present, it results in an automatic failure.
- Vehicle Readiness Monitors Are Not Cleared – The system checks various emissions components, and if one or more readiness monitors are incomplete, the car will fail.
- Possible Excessive Pollutants – A failing sensor or emissions component can cause higher-than-allowed carbon monoxide, nitrogen oxides, or hydrocarbons.
Common Check Engine Light Causes That Affect Emissions
- Faulty Oxygen Sensor – Causes incorrect air-fuel mixture readings.
- Bad Catalytic Converter – Prevents proper emissions filtering.
- Loose or Faulty Gas Cap – Triggers an EVAP system code.
- Mass Airflow Sensor (MAF) Issues – Affects fuel efficiency and emissions.
- Misfiring Engine – Results in increased emissions and possible damage.
How to Pass Emissions If the Check Engine Light Is On
1. Diagnose the Code with an OBD-II Scanner
- Get a free diagnostic scan at AutoZone, O’Reilly, or Advance Auto Parts.
- Use an OBD-II scanner at home to read the stored trouble code.
2. Fix the Underlying Issue
- If it’s a loose gas cap, tighten or replace it.
- For sensor issues, replace the oxygen sensor or mass airflow sensor as needed.
- If the catalytic converter is bad, a replacement is likely required.
3. Clear the Check Engine Light
- Use an OBD-II scanner to reset the code after fixing the issue.
- Disconnect the battery for 15 minutes (not always effective, and may reset monitors).
4. Drive for 50–100 Miles to Reset Readiness Monitors
- After resetting the check engine light, you need to complete a “drive cycle” so the system can confirm emissions components are working.
- Drive at varying speeds (highway and city) for about 50–100 miles before retesting.
What If the Check Engine Light Comes Back On?
If the light comes back on, the problem is not fixed. Get another diagnostic scan and repair the underlying issue before attempting another emissions test.
Final Thoughts
If your check engine light is on, you will likely fail the emissions test. Fix the underlying issue, reset the light, and complete a drive cycle before retesting. Ignoring the CEL will result in an automatic failure in most states.
FAQs
1. Can I just reset the check engine light before the test?
No, the test will check for readiness monitors, and if they are not complete, you will fail.
2. How long do I need to drive after clearing the CEL?
Typically 50–100 miles to reset the readiness monitors.
3. Will a loose gas cap cause emissions failure?
Yes, a loose or bad gas cap triggers an EVAP system error, which can cause failure.
4. Can I pass emissions with a pending trouble code?
Maybe. A pending code means an issue was detected but hasn’t turned on the CEL yet. Some states still fail for this.
5. Do all states fail emissions for a check engine light?
Most do, but some areas without strict emissions testing may allow passing if tailpipe emissions are within limits. Always check local regulations.
Also Check:
• Will My Car Pass Emissions with the Check Engine Light On?
• Will the Check Engine Light Reset Itself?
• Will a Loose Gas Cap Cause the Check Engine Light?
