A check engine light does not automatically fail you on an emissions test, but it can

A lit check engine light means your vehicle's onboard diagnostic system has detected a problem in the engine, emissions system, or transmission. Whether that light causes you to fail an emissions inspection depends on what triggered it and what your state's test procedure actually checks.

Some states test only tailpipe emissions — the gases coming out the back. Others run a full diagnostic scan that reads the codes stored in your vehicle's computer. If your state scans for codes, a check engine light almost certainly means you will not pass. If your state tests only tailpipe output, you might pass even with the light on, as long as the underlying problem has not yet degraded your actual emissions enough to exceed the limit.

The safest approach is to have the code read before you show up for inspection. A code reader (or a mechanic with one) will tell you what the problem is. From there, you can decide whether to fix it first or take the test and see what happens.

Key Takeaways

  • A check engine light indicates a stored diagnostic trouble code, which some states will find during an emissions test and some will not.
  • States that perform onboard diagnostic (OBD) scans will almost always fail you if a code is present, even if your tailpipe emissions are clean.
  • States that test only tailpipe emissions may pass you despite the light, depending on whether the underlying problem has degraded your actual emissions output.
  • You can have the code read at a parts store, dealership, or independent mechanic before your test to know what you are dealing with.
  • Some states allow you to clear a code and retest, while others require you to fix the problem and have the vehicle run for a certain number of drive cycles before retesting.

How your state's emissions test actually works

Emissions testing varies widely by state. Some states run a two-speed tailpipe test that measures only what comes out the exhaust. Others perform a full onboard diagnostic (OBD) scan that reads the computer codes directly. A few states do both.

If your state uses OBD scanning, the test machine plugs into your vehicle's diagnostic port (usually under the steering wheel) and reads whether any trouble codes are stored. A check engine light means at least one code is present. Most states that scan will fail you when ready if any code is found, regardless of your tailpipe emissions numbers.

If your state tests only tailpipe emissions, the test measures nitrogen oxides (NOx), hydrocarbons, and carbon monoxide at idle and at a higher engine speed. A check engine light does not directly cause a failure in this case — only if the underlying problem has degraded your emissions enough to exceed the state's limits.

You can find out which test your state uses by contacting your state's environmental agency or the facility where you plan to test. The facility staff can also tell you whether they have seen your specific code before and what typically happens.

What the code means and whether you need to fix it before testing

A check engine light can indicate anything from a loose gas cap to a failing catalytic converter. The only way to know is to read the code. Most auto parts stores (AutoZone, O'Reilly, NAPA, Advance Auto Parts) will read the code for free. You can also use a handheld OBD2 scanner, which costs $25 to $100, or visit a mechanic or dealership.

Once you have the code, you can research what it means. Common codes that trigger the light include P0420 (catalytic converter efficiency), P0171 (system too lean), P0300 (random misfire), and P0455 (large evaporative leak). Some codes indicate problems that will definitely cause you to fail an emissions test. Others indicate problems that may not affect your emissions output at all.

If your state performs OBD scanning, you will need to fix the problem and clear the code before you can pass, or at least before you can pass the diagnostic portion of the test. If your state tests only tailpipe, you can try the test as-is and see whether you pass based on your actual emissions numbers. Many people do pass with a check engine light on, depending on the underlying problem.

Clearing the code versus fixing the underlying problem

You can clear a check engine code by disconnecting the battery, using a code reader with a clear function, or having a mechanic do it. Clearing the code turns off the light. However, if the underlying problem is still present, the code will return — usually within a few days or a few hundred miles of driving.

Some states allow you to clear the code, drive the vehicle for a set number of drive cycles (often 40 to 100 miles of mixed driving), and then retest. If the code does not return during that driving period, you pass. This works only if the problem is intermittent or has resolved on its own.

Other states require you to fix the problem permanently before you can pass. If you clear the code and the problem is still there, the code will come back during the test or shortly after, and you will fail again. A mechanic can tell you whether the problem is something that might go away on its own (like a loose gas cap or a temporary sensor glitch) or something that requires actual repair.

Common check engine codes and emissions test outcomes

Some codes are almost certain to cause an emissions test failure. P0420 (catalytic converter efficiency below threshold) and P0430 (catalyst system efficiency below threshold) indicate your catalytic converter is not working properly — a core emissions control component. If you have either of these codes, you will almost certainly fail an emissions test in any state.

P0171 (system too lean) and P0174 (system too rich) indicate your engine is running with an incorrect fuel-to-air ratio. These codes often cause you to fail a tailpipe test because they directly affect what comes out the exhaust. P0300 (random misfire) and related misfire codes can also cause tailpipe failure because misfires increase emissions.

P0455 (large evaporative system leak) and P0456 (small evaporative system leak) indicate your fuel system is leaking vapors. These codes may or may not cause a tailpipe failure, depending on the size of the leak and your state's limits. However, if your state scans for codes, you will fail the diagnostic portion regardless.

Codes like P0128 (coolant thermostat not reaching operating temperature) or P0133 (oxygen sensor slow response) are less directly related to emissions output, but they still indicate a problem that your state's OBD scan will find. If your state tests only tailpipe, you might pass despite these codes.

What to do if you have a check engine light before your test

Start by having the code read. This costs nothing at most parts stores and tells you exactly what you are dealing with. Once you know the code, research it or ask a mechanic whether it is likely to cause an emissions test failure in your state.

If your state performs OBD scanning, assume you will fail if the code is present. Your options are to fix the problem, clear the code and drive the vehicle for the required number of drive cycles to see if it returns, or reschedule your test after you have fixed the problem.

If your state tests only tailpipe, you can attempt the test as-is. You might pass. If you fail, you will know the specific emissions that exceeded the limit, and you can decide whether to fix the problem or try again after clearing the code and driving for a set period.

Do not assume a check engine light means you will definitely fail. Do not assume it means you will definitely pass. Have the code read, find out what your state's test procedure actually checks, and make a decision based on that information.

Retesting after a repair or code clear

If you fix the problem, the code should not return. You can retest when ready after the repair is complete. If you clear the code without fixing the problem, most states require you to drive the vehicle for a certain number of drive cycles before retesting. A drive cycle typically means starting the vehicle, driving it through a mix of city and highway speeds, and letting it idle — usually 40 to 100 miles of normal driving.

Some states allow you to retest the same day if you clear the code and it does not return during a short test drive. Others require you to wait a set number of days. Check with your testing facility or your state's environmental agency for the specific rules in your area.

If you retest and the code returns during the test, you will fail again. This means the underlying problem is still present and needs to be repaired, not just cleared. A mechanic can help you determine whether the problem is something that can be fixed affordably or whether your vehicle may not be worth repairing.

Frequently Asked Questions

Will a check engine light automatically fail me on an emissions test?

Not automatically, but it depends on your state's test procedure. If your state scans your vehicle's onboard computer for diagnostic codes, a check engine light almost certainly means failure. If your state tests only tailpipe emissions, you might pass depending on what the underlying problem is and whether it has degraded your actual emissions output.

Can I clear the check engine light myself and then take the test?

You can clear the code, but if the underlying problem is still present, the code will return — usually within days or a few hundred miles of driving. Some states allow you to clear the code, drive for a set number of miles, and retest if the code does not return. Others require you to fix the problem permanently. Check your state's rules before you clear the code.

What if I fail the emissions test because of a check engine light?

You will need to fix the problem indicated by the code or clear the code and drive for the required number of drive cycles before retesting. A mechanic can read the code and tell you what repair is needed. Some repairs are inexpensive (like replacing a gas cap or oxygen sensor), while others (like catalytic converter replacement) are costly.

How much does it cost to fix a check engine light problem?

Costs vary widely depending on the code and the problem. A loose gas cap costs nothing to fix. An oxygen sensor might cost $100 to $300. A catalytic converter can cost $400 to $2,500 or more. Have the code read and get a repair estimate from a mechanic before deciding whether to fix it or explore other options.

Can I pass an emissions test if the check engine light comes back on during the test?

No. If the code returns during the test, you will fail. This means the underlying problem is still present and the code was not permanently cleared. You will need to repair the problem or have a mechanic diagnose why the code is returning so quickly after being cleared.