ABC smog is California's vehicle emissions classification system

California divides vehicles into three smog-check categories based on model year and fuel type: A, B, and C. Your vehicle's category determines which test it must pass and how often you test. The category is not about your car's condition — it is built into the vehicle's age and engine type the moment it rolls off the lot.

Category A covers the newest vehicles, Category B covers mid-range years, and Category C covers the oldest. A 2020 sedan might be Category A, a 2005 truck might be Category B, and a 1995 car might be Category C. Each category has different testing requirements because newer engines produce fewer emissions and older ones need more frequent checks.

You do not choose your category. The California Department of Motor Vehicles assigns it based on your vehicle's registration. When you renew your registration or buy a used car, the DMV letter or paperwork will tell you which category applies.

Key Takeaways

  • Your vehicle's ABC category is determined by its model year and fuel type, not by how well it runs or how you maintain it.
  • Category A vehicles (newest) test every two years, Category B vehicles test every two years, and Category C vehicles (oldest) test annually.
  • You can find your vehicle's category on your DMV registration renewal notice or by calling a smog station.
  • Failing a smog test means your vehicle does not meet California's emissions standards for that year, and you cannot register it until you repair it and retest.
  • Some vehicles are exempt from smog testing entirely, including electric vehicles and certain motorcycles.

How the three categories differ

Category A includes vehicles from roughly 2000 forward (the exact year changes annually). These cars have modern emission-control systems and produce the lowest pollution. They test every two years.

Category B includes vehicles from roughly the mid-1980s to 1999. These cars have older emission systems but still meet a baseline standard. They also test every two years, but the test standards are less strict than Category A.

Category C includes vehicles from 1975 to the mid-1980s. These are the oldest vehicles still on the road. They test every year because their engines produce more emissions. Some Category C vehicles are exempt if they meet specific conditions — for example, if they have been modified to run on alternative fuel or if they are registered as classic cars.

Diesel vehicles follow their own rules. Most diesel trucks and vans are Category C and test annually. Some newer diesel vehicles may fall into Category B depending on their model year and emissions equipment.

What happens when you test

When you take your vehicle to a smog station, the technician will look up your category using your license plate or VIN. The test itself measures your vehicle's tailpipe emissions — primarily nitrogen oxides, carbon monoxide, and particulate matter. The standards your car must meet depend on which category it falls into.

The test takes 10 to 30 minutes. The technician connects a probe to your exhaust pipe, runs the engine at idle and at higher RPMs, and records the readings. If your vehicle passes, you receive a certificate. If it fails, the station gives you a report showing which emissions exceeded the limit.

You can retest at any smog station, not just the one where you failed. Many people repair their vehicle and then go to a different station for the retest. There is no penalty for retesting — you only pay the test fee each time.

When you need to test based on your category

Category A and B vehicles test every two years. Your registration renewal notice will tell you the exact year you are due. If your notice says you need a smog test, you must complete it before your registration expires.

Category C vehicles test every year. If you own a Category C vehicle, you will need a smog certificate every 12 months to renew your registration.

New vehicle purchases have different rules. If you buy a used car, you typically have 30 days to get it smogged before you can register it in California. If you buy a new car from a dealer, it comes with a temporary registration that does not require an when ready smog test.

Some vehicles are exempt from testing altogether. These include electric vehicles, vehicles under 6,000 pounds that are model year 2000 or newer (in some cases), motorcycles, and vehicles registered as classic cars or street rods. Check your registration notice — it will state "smog exempt" if your vehicle does not need to test.

What to do if your vehicle fails

A failed smog test means your emissions are too high for your vehicle's category. You cannot register your car until you fix the problem and pass a retest. The smog station will give you a detailed report showing which emissions failed and by how much.

Common reasons for failure include a faulty oxygen sensor, a loose gas cap, a clogged air filter, or a malfunctioning catalytic converter. Some repairs are inexpensive; others cost several hundred dollars. A mechanic can read the report and tell you what needs fixing.

After you repair your vehicle, you can retest when ready. There is no waiting period. You pay the test fee again, but once you pass, you receive the certificate you need to complete your registration.

If repairs are very expensive and your vehicle is old, you may want to check whether the cost of repairs exceeds the vehicle's value. California has a Gross Polluter Abatement Program that can help in some cases, but the rules are strict and the program is limited.

Finding a smog station and scheduling

You can find a smog station through the California Bureau of Automotive Repair website, which maintains a searchable list of licensed stations. You can also search online for "smog check near me" and filter by location and hours.

Most smog stations do not require an appointment — you can walk in during business hours. Some busy stations may have a wait, especially near the end of the month or during registration renewal periods. Calling ahead to ask about wait times is a good idea.

The test fee is set by the state and does not vary by station. As of 2024, the fee is $29.75 for a basic test and $43.75 for a more complex test (used for vehicles with onboard diagnostic systems). You pay this fee whether you pass or fail.

Frequently Asked Questions

How do I know which category my vehicle is in?

Check your DMV registration renewal notice — it will list your vehicle's category. You can also call a smog station and give them your license plate or VIN, and they will tell you. The DMV website also has a lookup tool if you enter your vehicle information.

Can I test early, before my renewal is due?

Yes. You can test up to 60 days before your registration expires. The certificate is valid for 30 days after the test, so timing matters if you are testing very early. Most people test within a month of their renewal date to avoid confusion.

What if I move out of California?

Once you register your vehicle in another state, you no longer need California smog tests. If you move back to California later, your new vehicle will be assigned an ABC category based on its model year at that time.

Do I need a smog test to sell my car?

Not legally. California does not require a smog certificate to sell a used vehicle. However, a buyer may request one as part of the sale, and some dealers require it. The seller and buyer can negotiate this as part of the sale agreement.

What happens if I drive with an expired registration because I failed a smog test?

Driving with an expired registration is illegal, even if you are waiting for a retest. You can be cited and fined. If you fail a smog test, repair your vehicle and retest as soon as possible, or contact the DMV about a temporary extension if you have a documented repair appointment.