Can You Pass Emissions With Check Engine Light On?
If your vehicle has an emissions test coming up then you may be wondering if a check engine light affects the outcome of the test. There are several factors that can influence the outcome of your emissions test and many people aren’t aware of the main causes for a failed test.
So can you pass emissions with the check engine light on? The short answer is no. Ignoring this light will cause you to fail your emissions test and many states will not even perform an emissions test if your check engine light is on in your vehicle.
What Is the Emissions Test For?
An emissions test can be done in 5 minutes or less and involves a technician scanning your Vehicle Identification Number and then plugging in a cable to your vehicle’s On-Board Diagnostic system or OBD II.
It is a more accurate measurement of emissions and costs much less than previously used “tailpipe” tests. An emission test can normally be completed in under 5 minutes.
All vehicles made since 1996 are equipped with this onboard diagnostics system which is in place to detect and alert you to any emission problems your vehicle may be having before it becomes a larger problem.
The OBD will alert you to a problem so you can fix your vehicle and prevent unhealthy air and excessive pollutants from entering the air as well as helping you to avoid a shorter engine life and fuel wastage that could create an expensive mechanics bill.
Can You Pass Emissions With Check Engine Light On?
If your vehicle is from 1996 or newer then it will likely have OBD2 abilities then the check engine light will have a direct effect on if it passes the emissions test and often your car will not even be tested if the light is on in the first place.
Older vehicles may have a chance of passing, but this is only as the expectation for older cars is not as significant as newer models.
A check engine light can be an indicator of several issues with your vehicle which is why you won’t be able to pass your emissions test if it’s illuminated. It could reveal a simple issue, or could even be a sign of something much more serious which is why you will not pass the test.
How Can I Pass Emissions Testing?
In order to pass an OBD II emissions test, there are a few things that your vehicle must have:
- Your vehicle must have a functional ‘Check Engine Light’ and OBD port.
- The Check Engine Light must not be illuminated.
- Your vehicle must complete the OBD-II system monitors successfully.
Why Is My Check Engine Light Illuminated?
For many people, the check engine light can be an irritating and repetitive issue caused by their vehicle’s computer, but this only happens when one or more diagnostic trouble codes are stored in your car’s computer and the light will stay illuminated as long as this is there.
It can illuminate when there is a problem that could or is affecting your car’s emissions control system which is why your car will not pass an emissions test with this light illuminated.
If the light consistently stays on without having any performance issues then this means there is a permanent fault that will not be resolved without being fixed and you should find the cause of the problem before it creates issues with your engine management system, performance, and emission control.
When this happens, it means a vital component of your engine has a serious fault and is not safe to drive. The most common causes for a check engine light to illuminate include:
- Bad oxygen sensor.
- Loose gas cap.
- Spark plugs need replacing or are failing.
- The catalytic converter is faulty.
- Airflow sensor problems.
Check Engine Light Is Illuminated – Now What?
So if your check engine light is illuminated and you have an emissions test you need to pass then you have a few options to resolve the issue.
You should make sure your check engine light is not alerting you to a problem that requires your attention as this could lead to irreversible damage to your vehicle.
You will not be able to clear the check engine light or use an OBD2 scanner to reset the light as when your vehicle has its emissions test it will go through a self-check cycle. Also, most vehicle catalyst and EVAP monitors require time to run, meaning you may need to drive your vehicle for several days. This gives the OBD-II time to let any trouble codes or the check engine light reoccur.
Using an OBD-II Scanner
You will need to use a scanner to find out the root cause of your check engine light and fix the issue before you will be able to pass your emissions test. Use a scan tool on your vehicle to help locate the issue and guarantee you will pass your emissions test. Here’s how:
- Connect to the OBDLink app.
- Tap ‘Monitors’.
- In the top right corner, click the menu icon to choose the state that you live in.
- The screen will then automatically show a ‘Not Ready’ warning meaning that your vehicle would fail the emissions test, or a ‘Ready’ green check showing that it would pass.
This scanner will also allow you to carry out a pre-test on your vehicle to confirm if there are any issues or if it will pass the emissions test prior to taking it for testing.
If your vehicle is older than 1996 then you may not have a port for a scan tool so you may be lucky and get away with the light being illuminated.
FAQs
Is check engine light related to emissions?
The check engine light signals that the onboard diagnostics system has an issue related to the ignition, exhaust system, fuel, or emissions in your car.
What can I put in my gas tank to pass emissions?
If you want your car to run better or a bit of help to pass an emissions test then adding two cans of Sea Foam Motor Treatment to a low tank of fuel followed by driving 20 miles can help pass your emissions test.
What can cause emissions to fail?
Reasons your vehicle can fail the emissions test include an overdue oil change, an illuminated check engine light, worn spark plugs, loose gas cap, dirty air filters, and a rich air to fuel mixture.
Can you pass emissions with stored codes?
Your car or vehicle can pass the emissions test with a pending trouble code, so long as the check engine light is not illuminated and the necessary emission monitors are ready.
Conclusion
If your vehicle needs an emissions test then a check engine light is a guaranteed way to get instantly turned away but luckily there are ways around it that can enable your vehicle to pass without breaking the law.
Fixing the problem is often the best way to remove these warning lights and if you are unsure why your vehicle is having persistent faults then you should ask a local garage or mechanic for a professional opinion to make sure your vehicle is safe to drive,
So can I pass emissions with a check engine light on? The quick answer is no, for most vehicles made from 1996 onwards your check engine light will cause you to instantly fail your emissions test and be turned away.
By using a scan tool you can identify the cause of your problem and figure out the best course of action to solve it.
You may also check out these other VehicleFreak articles:
Can You Smog a Car With a Check Engine Light On?
Understand the relationship between your car’s check engine light and its emissions system to see why a lit-up dash is an automatic smog check failure.
Published Jul 2, 2025
In California, a vehicle with an illuminated check engine light will not pass a smog check. The light, officially known as the Malfunction Indicator Lamp (MIL), signals a problem within the vehicle’s engine or emissions system. Its presence results in an automatic failure of the inspection, and addressing the underlying issue is a necessary step before the vehicle can be tested.
Why a Check Engine Light Causes an Automatic Failure
The state’s smog check program, overseen by the Bureau of Automotive Repair (BAR), relies on the vehicle’s On-Board Diagnostics II (OBD-II) system. This system, mandatory in vehicles manufactured from 1996 onwards, continuously monitors the engine and emissions control components. When it detects a fault that could increase emissions, it triggers the check engine light and stores a specific Diagnostic Trouble Code (DTC) in its memory.
For vehicles model year 2000 and newer, the test is primarily electronic. The smog technician connects equipment to the vehicle’s OBD-II port to read data and confirm emissions components are working. An illuminated check engine light signifies an active fault, leading to an automatic failure. The state’s concern is the operational integrity of the entire emissions control system, not just tailpipe emissions.
Common Reasons for an Illuminated Check Engine Light
The reasons for a check engine light can range from simple fixes to more complex repairs. A common cause is a loose or faulty gas cap. The cap is a component of the evaporative emission control (EVAP) system, and if it doesn’t seal properly, fuel vapors can escape, triggering the light.
More significant issues frequently involve sensors or other parts that directly impact emissions. These include:
- A failing oxygen (O2) sensor, which measures the amount of unburned oxygen in the exhaust and sends data to the engine’s computer to optimize the fuel mixture.
- A malfunctioning mass airflow sensor, which measures the amount of air entering the engine.
- A failing catalytic converter, an expensive component that converts harmful gases into less harmful substances.
- Old spark plugs or faulty ignition coils, which can lead to engine misfires.
Required Steps Before Attempting a Smog Check
Before a smog check, you must address the cause of the check engine light. The first step is to have the vehicle’s Diagnostic Trouble Codes (DTCs) read. Many auto parts stores offer this service for free, or a qualified mechanic can perform a diagnostic scan to retrieve the specific code.
Once the code is known, a professional mechanic should diagnose the underlying problem. Simply clearing the code without performing a repair is not a solution; the OBD-II system will detect the persistent fault, and the check engine light will reappear. After the necessary repairs are completed and the code is cleared, the vehicle is still not immediately ready for a smog test.
The vehicle must complete a “drive cycle.” This involves driving the car under a variety of specific conditions, a mix of steady highway speeds and stop-and-go city driving, for at least 100 miles. This process allows the OBD-II system’s internal “readiness monitors” to run their self-tests and confirm the repairs were successful. A smog station cannot pass a vehicle if these monitors report as “incomplete” or “not ready,” which will happen if the codes were recently cleared without a proper drive cycle.
The Smog Check Inspection Procedure
When you arrive at a licensed smog station, the inspection begins with a visual check to confirm the check engine light bulb itself is functional. The technician will turn the key to the “on” position without starting the engine to ensure the light illuminates, and then they will start the engine to verify that the light turns off as it should.
Following the visual check, the technician connects a certified testing unit to your vehicle’s OBD-II data link connector, which is located under the dashboard. This equipment communicates with your car’s computer to scan for any stored or pending Diagnostic Trouble Codes. It also verifies the status of the vehicle’s readiness monitors. If there are no active codes and all readiness monitors are set to “ready,” the vehicle passes this portion of the inspection.
Check engine light – How long to wait until getting car smogged?
Hey, my car is due for a smog test within 5 days. but the check engine light came on 3 days ago. After investigating with the OBD reader, I found out it was an evap leak which seems to have been fixed by a new gas cap (old car. it happens). The check engine light is now off. However, I’ve been told that if your check engine light has been on in the last 2 weeks your car will not pass the smog check. I’ve also been told that I shouldn’t reset the system by disconnecting the battery as that will also cause a failure. So. how long after the check engine light is off can I take it in to get a smog test without fear of it failing because of a past light? Or. if I reset things via the battery disconnect method, would that be okay? I just want to get this done ASAP without any fear of wasting money on a smog check where I’ll fail for sure even though it actually is fine, and just because the system says there used to be an evap leak.
Do not disconnected the battery! It will cause all the emission monitors to clear out. Once that happens you’ll need to drive your car through an emissions Drive Cycle (usually takes a few days of driving; around 100 miles or so) to get them to set. Emission monitors are internal self-tests which the emissions computer performs to ensure the engine is running efficiently. Without all the emission monitors “Ready” or “Complete”, your car will fail the smog test. and yes, the check engine light will be off, but that won’t stop your car from failing.
So best bet in this particular case, and since the check engine light turned off by itself, we’d recommend going ahead with the smog check now. The check engine light only turns off when the emissions computer detects that a fault no longer exists. In fact the computer double and sometimes even triple checks the emissions trouble before deciding that in fact the fault is fixed. The light then turns off. The one thing we do recommend though is that once you arrive at the smog station make sure to advise the smog technician that you recently made repairs to your vehicle because the check engine light was on.
posted by SmogTips Support
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