Your Guide to California’s Smog Check Program

An Eye-Opening Look at Drowsy Driving

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 Vehicles Need Smog Checks, and When

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 year 1998 or later

• A smog check is required every other year or when ownership is transferred, no matter how old the vehicle is.

Most other vehicles are exempt from smog inspections

• That includes gasoline vehicles model year 1975 or older, diesel vehicles model year 1997 or older, diesel and natural gas vehicles that weigh more than 14,000 pounds, electric vehicles, and motorcycles.

What a Smog Check Involves Today


Originally, smog checks included tailpipe, visual, and functional inspections. Today, most gasoline vehicles from model year 2000 or newer and diesel vehicles from 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.

Why Vehicles Fail, and How to Fix It

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. However, newer vehicles (model year 2000 and newer) usually fail because their onboard diagnostics report a problem. Onboard diagnostic readings have largely replaced tailpipe inspections, so it’s important to know when they’re reporting a problem.

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Fault codes are present.
If there’s a malfunction in your vehicle’s emission system that the onboard diagnostics can detect—say, a misfire, a loose gas cap, or a cracked hose—a fault code will be issued, illuminating the Check Engine light and directing technicians to what needs fixing. An illuminated Check Engine light is an automatic smog check failure, so the underlying issue should be fixed beforehand.

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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 in the absence of a malfunction. That’s because they take time to complete if repairs are done or the battery is disconnected, and incomplete monitors will result in a smog check failure.

Not sure if your readiness monitors are completed? The Check Engine light will come on if they've found 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.

Smog check stations can provide guidance on the type of driving that’s needed for a vehicle to complete its readiness monitors after a repair or memory wipe.


How to help your vehicle's readiness monitors complete themselves


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Drive at steady speeds

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Avoid hard acceleration

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Keep the gas tank between ¼ and ¾ full

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Drive at least
50 miles, both city and highway

An important point: 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?

Auto 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.