Will a check engine light cause a smog check failure in California?

A check engine light (MIL) will most likely result in a failure. While California does not publish an explicit rule, in practice a lit MIL is a strong indicator of emissions non-compliance. Get the issue diagnosed and repaired before testing.

Can Darling Smog diagnose or repair my check engine light issue?

We can pull diagnostic codes to tell you what the system is seeing, but per state law we do not perform repair diagnoses. For diagnosis and repair, please visit a licensed repair facility. Once fixed, you can retest with us.

Do aftermarket parts cause a smog failure?

Aftermarket emissions-related parts must be OEM or CARB-approved. Parts without a valid CARB Executive Order (EO) number can cause an automatic failure in California.

What is a readiness monitor and why would it fail my test?

Readiness monitors are self-checks your vehicle runs to verify emissions systems are functioning. If monitors are not complete, the test can be marked incomplete or result in a fail. After repairs or clearing codes, complete a full drive cycle before testing.

How far should I drive to complete a drive cycle before a smog check?

Follow your manufacturer’s specific procedure when possible. As a general guideline, drive 150–200 miles over varied speeds and conditions to allow monitors to reset.

Can I retest at Darling Smog after fixing the issue?

Yes. Once the issue is resolved and the MIL is off with monitors set, you can return to Darling Smog for a retest.

Is there financial help for emissions-related repairs in California?

You may qualify for California’s Consumer Assistance Program (CAP), which offers income-based assistance for emissions-related repairs. Visit bar.ca.gov for details and eligibility.

What should I do the day of my smog check to improve my chances of passing?

Arrive with no MIL illuminated, ensure maintenance is up to date, and take a 15–20 minute drive to bring the engine to operating temperature. If you recently repaired the vehicle or cleared codes, complete the drive cycle first.

Top 5 Reasons Smog Checks Fail (And How to Avoid Them)

A smog check isn’t just another box to tick at the DMV — it’s your vehicle’s clean bill of health. Most drivers pass without a problem, but some hit a roadblock. At Darling Smog, we’ve seen just about every reason for a fail, and we’re here to help you sidestep them.

1. Check Engine Light (MIL) Is On

That little orange light on your dash? It’s more than a suggestion.
If it’s illuminated, your vehicle will most likely fail. California’s official language doesn’t spell it out in those words, but in practice, a lit MIL is one of the strongest indicators your car won’t make the cut.

What to do:

Bring your vehicle to a licensed repair facility for diagnosis. We can pull the diagnostic codes here at Darling Smog, but by law, we can’t diagnose or repair. Once a mechanic fixes the issue and the light is off, you’re welcome to retest with us.

2. Neglected Maintenance

A vehicle that’s overdue for service is more likely to put out higher emissions.
Old fluids, worn-out components, or skipped manufacturer service intervals can all push you toward a fail.

What to do:

Know your car’s current condition. Follow your manufacturer’s maintenance schedule. If any emissions-related parts have been replaced, make sure they’re OEM or CARB-approved aftermarket parts.

3. Incomplete Readiness Monitors

Your vehicle’s onboard computer runs self-checks — called readiness monitors — to confirm the emissions system is functioning.
If those monitors aren’t complete, the smog equipment can’t verify compliance.

When this happens:

After a battery replacement
After clearing a check engine light
After repairs

What to do:

Complete a drive cycle before testing. You can look up the exact procedure for your make and model, or follow the general guideline: drive 150–200 miles over a mix of speeds and conditions.

4. Illegal Modifications or Wrong Parts

California has strict rules about emissions-related equipment. If your vehicle has aftermarket parts that aren’t CARB-approved — or the wrong parts entirely — it’s an automatic fail. This applies to everything from catalytic converters to certain intake systems.

What to do:

If your car has been modified, make sure all parts meet California’s CARB standards. OEM equipment is always safe, but aftermarket parts must have an Executive Order (EO) number proving compliance.

5. Ignoring Known Issues

You know that sputter, rough idle, or “something’s off” feeling? It won’t magically disappear during a smog check. Small problems can lead to elevated emissions and a fail.

What to do:

Contact a trusted mechanic to run a diagnostic test and see if any codes are being pulled or if anything visually looks amiss.

If You Fail a Smog Check

Don’t stress. You can retest here at Darling Smog once the issue is fixed.
If repairs are expensive, you may qualify for help through California’s Consumer Assistance Program (CAP), which offers income-based financial aid for emissions-related repairs.

Top 5 Reasons Smog Checks Fail (And How to Avoid Them)

A smog check isn’t just another box to tick at the DMV — it’s your vehicle’s clean bill of health. Most drivers pass without a problem, but some hit a roadblock. At Darling Smog, we’ve seen just about every reason for a fail, and we’re here to help you sidestep them.

1. Check Engine Light (MIL) Is On

That little orange light on your dash? It’s more than a suggestion.If it’s illuminated, your vehicle will most likely fail. California’s official language doesn’t spell it out in those words, but in practice, a lit MIL is one of the strongest indicators your car won’t make the cut.

What to do:

Bring your vehicle to a licensed repair facility for diagnosis. We can pull the diagnostic codes here at Darling Smog, but by law, we can’t diagnose or repair. Once a mechanic fixes the issue and the light is off, you’re welcome to retest with us.

2. Neglected Maintenance

A vehicle that’s overdue for service is more likely to put out higher emissions.Old fluids, worn-out components, or skipped manufacturer service intervals can all push you toward a fail.

What to do:

Know your car’s current condition. Follow your manufacturer’s maintenance schedule. If any emissions-related parts have been replaced, make sure they’re OEM or CARB-approved aftermarket parts.

3. Incomplete Readiness Monitors

Your vehicle’s onboard computer runs self-checks — called readiness monitors — to confirm the emissions system is functioning.If those monitors aren’t complete, the smog equipment can’t verify compliance.

When this happens:

After a battery replacement

After clearing a check engine light

After repairs

What to do:

Complete a drive cycle before testing. You can look up the exact procedure for your make and model, or follow the general guideline: drive 150–200 miles over a mix of speeds and conditions.

4. Illegal Modifications or Wrong Parts

California has strict rules about emissions-related equipment. If your vehicle has aftermarket parts that aren’t CARB-approved — or the wrong parts entirely — it’s an automatic fail. This applies to everything from catalytic converters to certain intake systems.

What to do:

If your car has been modified, make sure all parts meet California’s CARB standards. OEM equipment is always safe, but aftermarket parts must have an Executive Order (EO) number proving compliance.

5. Ignoring Known Issues

You know that sputter, rough idle, or “something’s off” feeling? It won’t magically disappear during a smog check. Small problems can lead to elevated emissions and a fail.

What to do:

Contact a trusted mechanic to run a diagnostic test and see if any codes are being pulled or if anything visually looks amiss.

If You Fail a Smog Check

Don’t stress. You can retest here at Darling Smog once the issue is fixed.If repairs are expensive, you may qualify for help through California’s Consumer Assistance Program (CAP), which offers income-based financial aid for emissions-related repairs.

That little orange light on your dash? It’s more than a suggestion.
If it’s illuminated, your vehicle will most likely fail. California’s official language doesn’t spell it out in those words, but in practice, a lit MIL is one of the strongest indicators your car won’t make the cut.

A vehicle that’s overdue for service is more likely to put out higher emissions.
Old fluids, worn-out components, or skipped manufacturer service intervals can all push you toward a fail.

Your vehicle’s onboard computer runs self-checks — called readiness monitors — to confirm the emissions system is functioning.
If those monitors aren’t complete, the smog equipment can’t verify compliance.

Don’t stress. You can retest here at Darling Smog once the issue is fixed.
If repairs are expensive, you may qualify for help through California’s Consumer Assistance Program (CAP), which offers income-based financial aid for emissions-related repairs.