What happens during a smog check in California

The short version of a California smog check

A smog check is a short exam of your car's pollution controls. It usually takes about 30 minutes. BAR says a thorough inspection often runs 20 minutes or more. What the inspector does depends on one thing first, the model year of your car.

If your gas car or hybrid is a 2000 or newer, the inspector plugs a scanner into your car's computer. There is no probe in the tailpipe. If your car is a 1999 or older, the inspector also measures what comes out of the tailpipe. Diesels from 1998 on follow the newer, computer based path.

In 2025, 92.8% of California initial smog checks were the computer based kind. Only 7.2% were tailpipe tests. The rest of this guide walks through both, step by step, and shows what causes a fail.

Which test does your car get

California runs two testing tracks. They run on two different machines. The BAR-OIS machine handles the OBD style check. The BAR-97 handles the tailpipe check. Your model year decides which one you get.

OBD means on board diagnostics. It is the computer system every car has had since 1996. The state moved most cars to the OBD style check in March 2015, after AB 2289 became law in 2010. Hybrids joined the smog check program in January 2015.

Not sure your car needs a smog check at all? Motorcycles, electric cars, gas cars from 1975 and older, and diesels from 1997 and older do not need one. Gas and hybrid cars that are eight model years old or newer skip the renewal check, but they still pay a smog abatement fee. Our guide on smog check cost covers those fees.

A smog check, step by step

Here is what happens from the moment you hand over your keys.

1. The station looks up your car. The inspector works from DMV's record of your vehicle. The program area comes from your registration ZIP code. That is why an old address on file can stop a test. If it happens, call the DCA Consumer Information Center at (800) 952-5210.
2. The inspector logs in. Inspectors sign in to the testing system with a biometric palm scanner. This ties each test to a licensed person. The machine is a Windows PC running BAR's Cal-VIS software online, with a printer and barcode scanner.
3. The visual check. The inspector looks for the emissions parts your car should have.
4. The check engine light check. With the key on and the engine off, the light must come on. Then with the engine running, it must go out.
5. The scan or the tailpipe test. On a 2000 or newer car, a BAR certified device reads your car's computer. On a 1999 or older car, the inspector also runs the tailpipe test.
6. The functional checks. On older cars these include the EGR valve, ignition timing and the gas cap. On gas cars the inspector also checks for visible smoke.
7. The result. A pass sends an electronic certificate straight to DMV. A fail gives you a Vehicle Inspection Report that lists the reasons.

The whole inspection has to happen inside the station's approved work area. BAR's 2024 equipment rule also lets the system capture the location of an inspection and spot OBD simulators. Those simulators are a fraud tool. If you want to know more, read about illegal smog check stations.

What is checked on a 2000 and newer car

This is the test most drivers get. The inspector does not put a probe in your tailpipe. Instead the BAR-OIS machine does four jobs.

1. The visual inspection

The inspector looks over the parts that control pollution. The Smog Check Manual lists the PCV system, the evaporative system, the catalytic converter, EGR, ignition spark controls, fuel metering, air injection, computers and sensors. The inspector also looks for liquid fuel leaks and visible smoke from the exhaust.

2. The check engine light (MIL)

The check engine light must light up when you turn the key to on, and go out once the engine runs. It fails if it never lights at key on, if it stays lit with the engine running, or if it flashes. BAR is blunt about it. A vehicle will not pass a smog check with the check engine light on. Our guide on the check engine light and smog check goes deeper.

3. The OBD scan

The scanner reads stored trouble codes and the status of the car's self tests, called readiness monitors. Two extra checks sit on top of the scan.

• Permanent codes. Since July 1, 2019, cars from model year 2010 and newer fail if a permanent code is stored, even when the light is off. The one exception is a car that has completed 15 warm up cycles and 200 miles since the codes were last cleared.
• Modified software. Since July 19, 2021, illegally modified engine software fails the test. Software with a CARB Executive Order passes.

4. The readiness check

If too many monitors say "not ready," the car fails. This is the most common reason for a fail right after a battery change. Read our guide to readiness monitors for the allowed numbers and how long to drive.

One new wrinkle. Some 2026 and 2027 model year cars use a newer protocol called OBDonUDS. The current scanners cannot talk to it. Until new scanners are certified, the system skips the OBD test on those cars.

What is checked on a 1999 and older car

Older cars get a hands on exam. These tests run on the BAR-97 machine. The area you live in sets the tailpipe test.

In enhanced areas, most 1976 to 1999 cars drive onto a dynamometer. That is a set of rollers. The car runs under load, as if on a road, and the machine reads the exhaust. Basic and change of ownership areas use the simpler two speed idle test.

Functional checks on 1976 to 1995 cars

• EGR. The exhaust gas recirculation system has to work.
• Ignition timing. It must be within plus or minus 3 degrees of the maker's spec.
• Gas cap. The fuel cap must hold pressure.
• Low pressure fuel evaporative test (LPFET). This checks the fuel vapor system for leaks.

LPFET is required on all 1976 to 1995 model years except some cars. The exceptions are cars with no evaporative system, dual tank cars, diesel and gas fuel (CNG, LNG and LPG) cars, cars with no adapter, cars with an inaccessible factory canister, and single tanks over 50 gallons. The old fill pipe restrictor test was repealed in March 2015.

Cars from 1996 to 1999 also get an OBD scan on the BAR-97 machine.

How many cars get each test

The tailpipe test is fading. As old cars leave the road, fewer owners ever see it.

What makes cars fail each kind of test

The two tracks fail for different reasons. Overall, the OBD style check failed 7.78% of initial tests in 2025. The tailpipe check failed 17.88%. That gap makes sense. The tailpipe group is all older cars, and age is the biggest driver of failure.

For 2000 and newer cars, the OBD scan and the functional checks lead. Readiness failures are a big piece of that, at 4.11% of all tests. Visual failures are rare, near 1.3%.

For older cars, the tailpipe itself is the top cause, at 10.2% of tests. The deeper picture is in our guide on why cars fail smog.

After the test

If your car passes

The station sends your certificate to DMV by computer. You pay the state certificate fee of $8.25, and only on a pass. The station posts that fee separately, charges you what it pays, and adds no sales tax. A certificate is good for 90 days if you are selling or registering. Dealer certificates last up to two years.

If your car fails

You get a Vehicle Inspection Report with the reasons and any open safety recalls. You can retest at any station, unless your notice says STAR. In that case the retest that certifies the car must be at a STAR station. Go to our failed smog check guide for the full plan. If money is tight, read about CAP repair help.

Check your history

You can look up any car's smog history online at bar.ca.gov/inspection. Results show from midnight on the day after the test.

Pretests

A pretest is a practice run before the official test. Stations may offer them, and they also need a written estimate you approve first. BAR does not set smog check prices. Stations set their own and must give you that estimate.