Can You Pass a Smog Check With the Check Engine Light On?

No. If your check engine light is on because the vehicle’s emissions system has detected a problem, the vehicle will not pass a California Smog Check.

The California Bureau of Automotive Repair specifically advises drivers to have a check engine light repaired before getting a Smog Check.

If your check engine light is illuminated, it is usually better to diagnose and repair the problem before bringing the vehicle in for its official inspection.

Why Does the Check Engine Light Cause a Smog Check Failure?

The check engine light, also known as the Malfunction Indicator Lamp (MIL), is part of your vehicle’s onboard diagnostic system.

When the vehicle’s computer detects certain engine or emissions-related problems, it may:

• Store a diagnostic trouble code

• Turn on the check engine light

• Record information about the malfunction

During a California Smog Check, the inspection system checks the vehicle’s onboard diagnostic system on most 2000 and newer vehicles. BAR’s inspection standards include specific pass/fail requirements related to the malfunction indicator lamp.

If the emissions-related MIL is commanded on, the vehicle cannot pass the OBD portion of the inspection.

Should I Clear the Check Engine Light Before a Smog Check?

Simply clearing the codes is usually not a good strategy.

Disconnecting the battery or using a scan tool to erase the codes may turn the check engine light off temporarily, but it can also reset the vehicle’s readiness monitors.

Readiness monitors are built-in self-tests that the vehicle performs to confirm that its emissions systems are operating properly. BAR explains that these monitors often need to run again after the battery has been disconnected or after emissions-related repairs.

That means a vehicle with freshly erased codes may still be unable to pass its Smog Check because the required monitors have not completed.

What Are Readiness Monitors?

Readiness monitors are tests performed automatically by the vehicle’s onboard diagnostic system.

They monitor systems such as:

• Catalytic converter operation

• Oxygen sensors

• Fuel system

• Misfire detection

• Evaporative emissions system

• Other emissions-related components

After repairs or a battery disconnect, the vehicle may need to be driven under certain conditions before these monitors become ready again.

BAR advises drivers who recently disconnected or replaced their battery to drive the vehicle for a period of time before getting a Smog Check so the readiness monitors have an opportunity to reset.

California Readiness Monitor Rules Have Changed

California has recently tightened its readiness-monitor requirements.

BAR adopted updated regulations requiring applicable readiness monitors to be completed before a vehicle can pass, with exceptions for certain vehicles that cannot feasibly set particular monitors. The updated rules began taking effect October 1, 2025.

Because requirements can depend on the vehicle, simply assuming that one incomplete monitor is always acceptable is no longer a good approach.

If you recently had repairs performed or cleared trouble codes, it is better to make sure the vehicle has completed its required monitors before testing.

How Long Should I Drive After Clearing Codes?

There is no single mileage or number of days that works for every vehicle.

Different manufacturers use different driving conditions to complete their readiness monitors. BAR notes that monitor completion can depend on factors such as:

• Vehicle speed

• Engine temperature

• Engine load

• Pressure

• Driving conditions

Some monitors may complete during normal driving, while others require more specific operating conditions.

The owner’s manual or manufacturer-specific service information may provide additional guidance for your vehicle.

What Should I Do If My Check Engine Light Is On?

Before going for a Smog Check:

1. Have the vehicle scanned for diagnostic trouble codes.

2. Diagnose the cause of the check engine light.

3. Repair the underlying problem.

4. Clear the codes only after the appropriate repair.

5. Drive the vehicle until its required readiness monitors have completed.

6. Then bring the vehicle in for its Smog Check.

This gives you a much better chance of passing than simply erasing the warning light immediately before the test.

Common Reasons a Check Engine Light Comes On

A check engine light can be triggered by many different issues, including:

• Faulty oxygen sensors

• Catalytic converter problems

• Misfires

• Evaporative emissions leaks

• Fuel-system problems

• Sensor or wiring issues

• Emissions-control component failures

The trouble code is a starting point for diagnosis, but it does not always mean the part named in the code needs to be replaced.

Will My Car Fail If the Check Engine Light Is Off?

Not necessarily.

A vehicle can still fail a Smog Check even when the check engine light is not illuminated.

California Smog Check inspections may include:

• A visual emissions-system inspection

• Functional checks

• An onboard diagnostic inspection

• A tailpipe emissions test on applicable older vehicles

Most 2000 and newer vehicles are inspected using BAR’s onboard diagnostic inspection system, while many 1999 and older vehicles use the BAR-97 emissions inspection system.

So having the check engine light off is important, but it does not automatically guarantee a passing inspection.

Can Smog Tech Pros Check My Vehicle?

Yes.

Smog Tech Pros performs California Smog Check inspections for customers throughout Rancho Cordova and the surrounding Sacramento area.

We are a STAR Certified Smog Check station, and walk-ins are welcome.

If you're unsure whether your vehicle is ready for its Smog Check, give us a call before coming in.

Frequently Asked Questions

Can I pass smog if I just erased the check engine light?

Not necessarily. Erasing the codes can reset the vehicle’s readiness monitors. If the required monitors have not completed, the vehicle may not pass the OBD portion of the inspection.

Will disconnecting the battery help me pass smog?

No. Disconnecting the battery can clear diagnostic information and reset readiness monitors. BAR advises giving the vehicle time to complete those monitors again before a Smog Check.

What if my check engine light comes back on?

If the light comes back on, the underlying problem likely still exists. The issue should be diagnosed and repaired before the official Smog Check.

Does Smog Tech Pros require an appointment?

No. Walk-ins are welcome.

Is Smog Tech Pros STAR Certified?

Yes. We can inspect both regular Smog Check vehicles and vehicles directed to a STAR station.

Smog Check in Rancho Cordova

Need a Smog Check?

Smog Tech Pros
10751 Folsom Blvd
Rancho Cordova, CA 95670
916-638-4700

STAR Certified • No Appointment Necessary • Free Retest Included

We will match or beat any coupon in Rancho Cordova.

Restrictions apply.