Why Did My Car Fail a Smog Check?

If your vehicle failed a California Smog Check, the reason should be listed on the Vehicle Inspection Report you received from the station. California BAR says that after every inspection, the station must provide a report showing whether the vehicle passed or failed and the inspection results.

The most common reasons for failure include a check engine light, incomplete readiness monitors, emissions-system problems, failed functional checks, or a visual inspection problem.

1. The Check Engine Light Is On

If the check engine light is on because the vehicle’s emissions system has detected a problem, the vehicle will not pass the Smog Check.

California BAR specifically recommends having the check engine light repaired before getting the vehicle tested.

Common causes can include:

• Oxygen sensor problems

• Misfires

• Catalytic converter issues

• EVAP system leaks

• Fuel-system problems

• Sensor or wiring problems

The diagnostic trouble code is a starting point for determining the cause, but it does not always mean the component named in the code automatically needs replacement.

2. The Readiness Monitors Are Not Complete

This has become especially important in California.

Readiness monitors are self-tests performed by your vehicle’s onboard diagnostic system to verify that emissions-control systems are functioning properly.

If you recently:

• Disconnected the battery

• Replaced the battery

• Cleared diagnostic trouble codes

• Had emissions repairs performed

some of the vehicle’s monitors may show not ready.

California’s updated OBD readiness rules began taking effect October 1, 2025 and generally require applicable readiness monitors to be completed for the vehicle to pass, subject to BAR-approved exceptions for certain vehicles.

Don't Clear Codes Right Before Your Smog Check

Erasing codes may turn off the check engine light temporarily, but it can also reset the readiness monitors.

BAR recommends driving the vehicle after a battery disconnect or replacement so the monitors have an opportunity to complete again.

There is no single mileage that works for every vehicle because different monitors require different operating conditions.

3. The Vehicle Failed the Visual Inspection

A California Smog Check can include a visual inspection of the emissions-control system.

An inspector may check required emissions equipment to make sure it is present and properly connected.

Depending on the vehicle, a visual failure could involve emissions-related equipment that is:

• Missing

• Disconnected

• Modified improperly

• Damaged

• Not approved for use on that vehicle

Aftermarket emissions parts may need to meet applicable California requirements.

4. The Vehicle Failed a Functional or OBD Test

For most 2000 and newer vehicles, California uses the BAR On-Board Diagnostic Inspection System.

The inspection obtains emissions-related information directly from the vehicle’s computer. Older vehicles may instead receive a tailpipe emissions test using BAR-97 equipment.

A vehicle can fail the OBD portion because of things such as:

• Emissions-related diagnostic trouble codes

• Malfunction indicator lamp problems

• Incomplete readiness monitors

• Communication problems

• Certain emissions-system faults

BAR's July 2026 statewide report shows that OBD-related failures account for a significant portion of initial Smog Check failures on OIS-tested vehicles.

5. An Older Vehicle Failed the Emissions Test

Many 1999 and older vehicles are inspected using California's BAR-97 system, which measures tailpipe emissions.

Older vehicles may fail because measured emissions exceed California standards.

Possible causes can include:

• Catalytic converter deterioration

• Fuel mixture problems

• Ignition problems

• Engine wear

• Oxygen sensor problems

• Other emissions-control failures

A proper diagnosis is important before replacing parts.

6. The Vehicle Has Modified Software

Modified engine or emissions software can also cause a Smog Check failure.

BAR advises owners to restore vehicle software to the manufacturer’s original configuration or a version approved by the California Air Resources Board when applicable.

This can be particularly relevant for vehicles with aftermarket tuning or performance modifications.

What Should I Do After My Vehicle Fails?

Start with your Vehicle Inspection Report.

The report identifies the inspection result and can help determine what portion of the test caused the failure.

California BAR recommends:

1. Review the Vehicle Inspection Report.

2. Ask the Smog Check station for clarification if necessary.

3. Complete the required repairs.

4. Have the vehicle inspected again.

If your DMV notice requires a STAR station, the retest must be performed at an appropriately certified STAR station.

Don't Replace Parts Without Diagnosing the Problem

A trouble code does not necessarily mean you should immediately replace the component mentioned in the code.

For example, a catalytic-converter-related code can have several possible causes.

Proper diagnosis can help you avoid spending money on unnecessary parts.

Can I Drive the Vehicle Before Retesting?

In many cases, yes.

If repairs were recently completed or diagnostic codes were cleared, the vehicle may need normal driving or a manufacturer-specific drive cycle before its readiness monitors complete.

Readiness monitor completion can depend on conditions such as:

• Vehicle speed

• Engine temperature

• Engine load

• Pressure

• Driving conditions

BAR notes that some vehicles can take longer than others to complete certain monitors.

What If I Can't Afford the Repairs?

California BAR operates a Consumer Assistance Program that may provide eligible motorists with financial assistance for emissions-related repairs. Vehicle retirement may also be available for qualifying vehicles and owners.

Eligibility requirements apply.

Smog Tech Pros Includes a Free Retest

If your vehicle does not pass its Smog Check at Smog Tech Pros, our smog service includes a free retest, subject to applicable restrictions.

That means after you correct the problem, you can return for the retest without paying our regular inspection price again under the terms of the offer.

Why Rancho Cordova Drivers Choose Smog Tech Pros

Smog Tech Pros offers:

• STAR Certified inspections

• No appointment necessary

• Walk-ins welcome

• Fast service

• Competitive pricing

• Free retest included

• Online coupon offers

• Convenient Folsom Boulevard location

We Will Match or Beat Any Coupon in Rancho Cordova

If you find a qualifying smog coupon from another Rancho Cordova location, bring it to us.

We will match or beat any coupon in Rancho Cordova.

Restrictions apply.

Frequently Asked Questions

Why did my car fail smog even though the check engine light is off?

A vehicle can still fail because of incomplete readiness monitors, a visual inspection issue, an OBD problem, a functional inspection issue, or excessive emissions on an applicable older vehicle.

Can I just erase the trouble codes and retest?

That usually isn't a good idea. Clearing codes can reset readiness monitors, which may prevent the vehicle from passing until those monitors complete again.

How do I know exactly why my car failed?

Check the Vehicle Inspection Report provided after the test. It shows the inspection results and whether the vehicle passed or failed.

Do I have to return to the same smog station?

California generally allows you to get the vehicle inspected again after repairs, but if DMV requires a STAR station, you need to use a STAR-certified station.

Does Smog Tech Pros offer a free retest?

Yes. Free retest included, subject to applicable restrictions.

Failed a Smog Check in Rancho Cordova?

Bring your vehicle to:

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.