Check engine light and smog check in California

Will a check engine light fail a smog check

Yes. In California, a car with the check engine light on does not pass. BAR says so in plain words. It does not matter if the car runs fine. It does not matter if the light came on for a small reason. The light, also called the MIL, is the car telling the inspector it found a problem in its pollution controls.

This is the most common way modern cars fail. In 2025 the OBD part of the smog check failed 714,956 cars. That is 6.84% of every 2000 and newer car tested, and it made up about 88% of all the failures on that kind of test.

Good news first. A light is a clue, not a verdict. Most causes can be fixed, and some are covered by a free warranty. Read on for what the inspector looks for and the order to do things in.

How the inspector checks the light

The test has two parts. The first is a bulb check. With the key on and the engine off, your dashboard lights up. The check engine light has to come on too. That proves the bulb works. Then the engine runs, and the light has to go out.

This means a burned out bulb or a bulb that someone disabled also fails. A light that was unplugged to hide a problem will not pass.

What the OBD scan looks at

After the bulb check, the inspector connects a BAR certified scanner to the data port under your dash. On 2000 and newer cars, that scan is the heart of the test. It reads four things.

• The light itself. Is the MIL commanded on?
• Stored codes. A stored fault code points to a failing part.
• Readiness monitors. The car runs its own self tests. If too many have not finished, the car fails. See our readiness monitors guide.
• Permanent codes. This is an extra layer, explained below.

For the full walk through of the visit, read what happens during a smog check.

Which cars fail the OBD check most

Age matters a lot. Parts wear out, and sensors drift. The chart shows the 2025 OBD fail rate for each model year from 2000 to 2025.

The oldest cars on this track, the 2000 model year, failed the OBD part 14.7% of the time. By model year 2017 the number fell to 3.7%. The newest model years tick up a little. Even new cars are not immune.

Across all model years the rate has barely moved since 2019. It has hovered between about 6.8% and 7.6%.

What usually turns the light on

BAR and CARB measure this at the roadside, where officers scan failing cars. The most common code families tell you where trouble starts.

In plain words, nearly 4 in 10 failing cars had an emission control code. That bucket includes the evaporative system (EVAP), the EGR valve and the catalytic converter. A loose gas cap sits in the EVAP family. A second group involves the air and fuel mix. A third group is misfires. Together those three families make up about 80% of the codes in that sample.

Other things BAR and the Referee point to include wrong or aftermarket catalytic converters. For example, a 2000 to 2007 Honda Accord built to the SULEV standard needs the factory converter. They also name modified software, which now fails the test on its own.

The trap that catches people who clear the code

It is tempting to erase the code with a scanner, or to disconnect the battery, so the light goes away. Do not do this right before a test. It causes two separate fails.

1. It resets your readiness monitors. The car has to drive through its self tests again. BAR says to drive one to two weeks after a battery disconnect or replacement before you test.
2. It does not erase permanent codes. On cars from model year 2010 and newer, a permanent code stays stored even when the light is off. It goes away only after the car confirms the fault is fixed. The state fails the car if a permanent code is stored, unless the car has completed 15 warm up cycles and 200 miles since the codes were last cleared.

Short version. Fix the real problem first. Then drive the car normally until the monitors finish. Then test.

What to do when the light is on, in order

1. Do not clear the code. Write down the code or have a shop read it.
2. Check your warranty first. Federal law and California rules can make the maker pay. See the table below.
3. Get a written estimate. Smog repairs cost real money. The average reported emissions repair in 2025 was $756.68, so ask questions and get it in writing.
4. Repair the cause. Pick a licensed smog repair shop. If your notice says STAR, the repair shop and the retest must follow that rule.
5. Drive it. Give the monitors time to finish. A week or two of normal driving is what BAR suggests after a reset.
6. Retest. If the light is out and the monitors are ready, you should pass.

You can call the CARB helpline at (800) 242-4450 for recalls and warranty questions.

What a fix usually costs

Stations log the repairs they do on failed cars in BAR's system. That log is a floor, since some repairs never get logged. Still, it gives the best picture we have. In 2025 stations logged repairs on 46,033 vehicles. The average was $756.68.

Roughly 4 in 10 of those repairs ran over $650. Those jobs averaged $1,525. That is why the warranty and the help programs matter. If your light is on and the car is young, call the dealer before a shop.

Cases that trip people up

You are buying a car with the light on

A seller has to hand over a valid smog certificate when a car is sold. A certificate is valid for 90 days. A car with the light on cannot get one, so a seller with a lit light has a repair to do first. Cars that are four model years old and newer are an exception, and the buyer pays an $8 transfer fee instead.

You changed your software or engine tune

Since July 19, 2021, illegally modified software gets its own failure. It shows up as a "Modified Software" result. A tune that has a CARB Executive Order passes. A tune that does not have one fails, even with the light off.

You put on a different catalytic converter

Wrong or aftermarket converters are a known cause of OBD fails. If your car was built to a strict California standard, the factory part may be the only one that works. The Referee station program handles disputes over converter labels. In 2025 it did 722 catalyst label inspections and 262 OBD monitor issue inspections.

The light comes on and off

An on and off light can be hard to catch. The test only needs the light to be on during the test or a code to be stored. If your car has thrown a code, have a shop find out why before your test date, not on it.

Is it getting better

A little. Here are the monthly OBD fail rates for 2026 so far. Since May they have run lower than the 6.84% for all of 2025.

If the repair costs more than you can pay

You have three programs to look at. They are covered in detail in our other guides.

• CAP repair help. Income eligible owners can get up to $1,450 toward repairs on a 1996 or newer car. See CAP repair assistance.
• Vehicle retirement. The state buys some failing cars for $1,350 to $2,000.
• The repair cost waiver. After at least $650 of eligible repairs and a second fail, the Referee may postpone the requirement for up to two years.

The full ladder is in our failed smog check guide.