Smog Check Plus is an enhanced emissions test that checks your vehicle's onboard diagnostic system

Smog Check Plus is California's more thorough version of the standard smog check. Instead of just testing your tailpipe emissions, it reads the data stored in your vehicle's onboard diagnostic (OBD) system — the computer that monitors your engine and emissions equipment. This test catches problems that a basic tailpipe test might miss, especially issues with your catalytic converter, oxygen sensors, and other emissions-control parts.

Not every vehicle gets a Smog Check Plus. Your car receives this test only if it fails the standard smog check first, or if it's a 1996 or newer model that hasn't passed a smog check in the past two years. The test costs more than a standard check — typically $15 to $20 extra — but it gives technicians a clearer picture of what's wrong with your emissions system.

Key Takeaways

  • Smog Check Plus reads your vehicle's onboard computer to find emissions problems that a basic tailpipe test cannot detect.
  • You only receive this test if you fail a standard smog check or if your 1996-or-newer vehicle hasn't passed one in two years.
  • The test costs between $15 and $20 more than a standard smog check, depending on your test station.
  • If your vehicle passes Smog Check Plus, you get a certificate that satisfies California's emissions requirement for registration.

When your vehicle gets tested with Smog Check Plus

Your car enters the Smog Check Plus program in two main situations. First, if you take your vehicle for a standard smog check and it fails, the technician will run the Plus test to identify the specific problem. Second, if your vehicle is a 1996 model or newer and your last passing smog check was more than two years ago, the state requires the Plus test instead of a standard check.

Vehicles older than 1996 never receive Smog Check Plus, even if they fail a standard test. Those older cars use a different testing method because they don't have the onboard diagnostic systems that newer vehicles have. If you own a pre-1996 vehicle and it fails, you'll work with a technician to fix the emissions problem and then retest using the standard method.

What the onboard diagnostic test actually measures

When a technician runs Smog Check Plus, they connect a scanner to your vehicle's OBD port — usually located under the steering wheel or dashboard. This scanner reads the data your engine's computer has been collecting about how your emissions systems are performing. The test looks for stored trouble codes, which are digital records of problems your vehicle has detected.

The scanner checks whether your catalytic converter is working, whether your oxygen sensors are functioning, whether your fuel system is sealed properly, and whether your engine is running too rich or too lean. It also verifies that your vehicle's emissions monitoring system itself is working — sometimes a vehicle will fail because the monitoring system has a problem, not because the emissions are actually bad.

This is why Smog Check Plus catches things a tailpipe test misses. A tailpipe test only measures what's coming out of your exhaust right now. The OBD test shows what your vehicle has been doing over time and flags parts that are beginning to fail before they create visible emissions problems.

How to prepare your vehicle for Smog Check Plus

Before you take your car to a test station, drive it normally for at least 100 miles. Your vehicle's onboard diagnostic system needs time to run its self-checks and store any trouble codes. If you just had your battery disconnected or replaced, or if you recently cleared your vehicle's trouble codes, you may need to drive even longer — sometimes 200 miles or more — for the system to complete its monitoring cycle.

Make sure your gas cap is tight and in good condition. A loose or damaged cap can trigger a trouble code related to your fuel system, which will cause your vehicle to fail. If your check engine light is on, have a technician scan the codes before your smog check appointment. Knowing what the problem is ahead of time lets you decide whether to fix it before testing or to proceed with the test and see what the results show.

Bring your vehicle registration and proof of ownership to the test station. You'll need to show that you own the vehicle and that it's registered in California. Some test stations also ask for your vehicle's VIN (vehicle identification number) to verify which emissions standards your car must meet.

What happens if your vehicle fails Smog Check Plus

If your vehicle fails, the test station will give you a detailed report showing which trouble codes were found and what they mean. Common failures include a faulty catalytic converter, a bad oxygen sensor, a leak in the fuel system, or a malfunctioning emissions control valve. The report will tell you specifically what system is causing the problem.

You then have the choice to repair the vehicle yourself, take it to a mechanic, or use a referee station if you believe the test was wrong. A referee station is a state-certified facility that can retest your vehicle at a lower cost if you think there was an error. You must request a referee retest within 30 days of your original test failure.

Once you've fixed the problem, you can return to any smog check station for a retest. You don't have to go back to the same station. When your vehicle passes, you'll receive a certificate that you can take to the DMV or submit online to complete your vehicle registration.

The difference between Smog Check Plus and a standard smog check

A standard smog check measures only the emissions coming from your tailpipe at the moment of testing. The technician runs your engine and collects samples of what's being expelled. This test is faster and costs less — usually $30 to $50 depending on your vehicle type and location.

Smog Check Plus goes deeper by reading your vehicle's computer history. It takes longer and costs more, but it finds problems that haven't yet created visible emissions. This is especially useful for vehicles that are borderline — cars that might pass a tailpipe test on one day but fail on another depending on driving conditions, fuel quality, or weather.

Think of it this way: a standard smog check is a snapshot of your emissions right now. Smog Check Plus is a record of your emissions over the past several weeks or months, plus a diagnostic check of all the parts that control those emissions.

Costs and where to get tested

Smog Check Plus costs vary by test station and by vehicle type. A standard smog check typically runs $30 to $50. Adding the Plus test usually costs an additional $15 to $20. Some stations charge a flat rate for both tests combined, which might be $50 to $70 total. Call ahead to ask what your station charges for your specific vehicle.

You can get tested at any California-certified smog check station. These include independent smog shops, some repair shops, and a few chain automotive service centers. To find a station near you, search online for "smog check station near me" or visit the Bureau of Automotive Repair website, which maintains a list of certified stations by county.

If you fail and want a second opinion, you can request a retest at a different station. If you believe the test was unfair or incorrect, you can request a referee retest at a state-certified referee station, which costs less than a regular retest.

Frequently Asked Questions

Do I have to get Smog Check Plus or can I just do a standard smog check?

If your vehicle is required to have the Plus test — because it failed a standard check or because it's 1996 or newer and hasn't passed in two years — then you must do the Plus test. You cannot choose to do only a standard check instead. If your vehicle is older than 1996 and passes a standard check, you will never be required to do Smog Check Plus.

What if my check engine light is on when I go for Smog Check Plus?

Your vehicle will likely fail if your check engine light is on, because that light means your onboard diagnostic system has detected a problem. However, you can still take the test to find out what the problem is. Once you know, you can decide whether to fix it before retesting or to use a referee station if you think the light is a false alarm.

How long does Smog Check Plus take?

The test itself usually takes 15 to 30 minutes, depending on how quickly your vehicle's computer responds and whether any trouble codes need to be investigated further. Waiting time at the station can add another 30 minutes to an hour depending on how busy they are. Call ahead to ask about wait times.

Can I pass Smog Check Plus if my vehicle has a trouble code stored?

Not usually. If your onboard diagnostic system has stored a trouble code — even if your vehicle seems to run fine — you will fail the test. The code means your vehicle has detected a problem with an emissions system, and California requires that problem to be fixed before you can pass.

What if I fix my vehicle but it still fails Smog Check Plus?

After you repair your vehicle, you need to drive it for at least 100 miles before retesting so your onboard diagnostic system can run its self-checks again. If you retest too soon, the system may not have completed its monitoring cycle and you could fail again even though the repair was successful. If you've driven enough and still fail, ask the technician to print the trouble codes so you know what to fix next.