Your guide to California's Smog Check Program

Vehicle emissions are a significant source of smog in California, which is why the state adopted its Smog Check Program in 1984. The program identifies and requires repairs for vehicles that emit excess smog-producing pollution. Smog checks take place when vehicle owners renew their registration or transfer their vehicle to someone other than a family member. Not every vehicle needs smog checks, though, and the technical details of smog checking have changed significantly over time.

Which cars need smog checks

Gasoline, hybrid, light natural gas, and flex-fuel vehicles, model year 1976 or later

  • A smog check is required every other year, unless the vehicle is six or fewer model years old.
  • A smog check is also required to transfer ownership, unless the vehicle is four or fewer model years old.

Diesel passenger cars and light trucks, model 1998 or later

  • A smog check is required every other year or when ownership is transferred.

Most other vehicles are exempt from smog inspections

  • That includes gas vehicles model year 1975 or older, diesel vehicles model 1997 or older, electric vehicles, and motorcycles.

What a smog check involves

Originally, smog checks included tailpipe, visual, and functional inspections. Today, most gasoline vehicles model year 2000 or newer and diesel vehicles model 1998 or newer no longer require a tailpipe inspection, where exhaust is analyzed while the vehicle is driven on a stationary dynamometer. That’s because modern cars are cleaner and have sophisticated onboard diagnostic systems that monitor and identify emission-control problems.


Rather than analyzing exhaust for excess emissions, technicians can connect directly to a modern car’s onboard diagnostics. If there is a problem with a vehicle’s emissions, a technician can read the assessment and quickly get an idea of what repairs might be required.

The ways vehicles fail smog checks

  • Too many pollutants emitted from the tailpipe

    Older vehicles that receive tailpipe inspections can still fail a smog check the old-fashioned way by producing too many pollutants when driven on a dynamometer. This is most common in model years prior to 2000; onboard diagnostic readings have largely replaced tailpipe inspections for vehicles newer than that.
  • Emission system fault codes are present

    If there’s a malfunction in your vehicle’s emission system that the onboard diagnostics can detect—say, a cracked hose or a loose gas cap—a fault code will be issued, illuminating the Check Engine light and directing technicians to what needs fixing. A Check Engine light is an automatic smog check failure.
  • Readiness monitors aren't complete

    Modern vehicles continuously complete self-tests called “readiness monitors” to track components like ignition and fuel evaporative systems. They generate fault codes if they detect a problem, but can also fail a smog check if repairs were recently done or the battery is disconnected and the monitors haven't had a chance to run.

How to know if your readiness monitors haven't finished

The Check Engine light will come on if your vehicle's readiness monitors find a problem, but there's no dashboard indicator if they've completed successfully. In many newer vehicles, if the driver turns the ignition key to the “on” position without starting the car (the “accessory” position), the Check Engine light should stay on. If it flashes after 15 seconds, the readiness monitors aren’t complete.

How to help your vehicle complete its readiness monitors

Smog check stations can provide guidance on the type of driving needed for a vehicle to complete its readiness monitors after a repair or memory wipe. In general, you'll want to follow these steps:


  • Drive at steady speeds
  • Avoid hard acceleration
  • Keep the gas between a quarter and three-quarters full
  • Drive at least 50 miles, both city and highway

Readiness monitors mean you can't hide fault codes

Fault codes can’t be hidden by erasing them or disconnecting the battery; the readiness monitors must be completed to determine if the fault still exists, or the vehicle won't pass a smog check.


If a vehicle’s readiness monitors continually won’t complete, it may mean that the vehicle has a malfunctioning component or sensor that needs to be adjusted or replaced.

Need a smog-related repair?


AAA Club members receive guaranteed repairs and service from AAA Approved Auto Repair facilities. If your Check Engine light is on or your vehicle isn't passing smog checks for another reason, you can get peace of mind by having your vehicle worked on at a shop you can trust. Find an Approved Auto Repair facility near you.