On-Board Diagnostic II (OBD II) Systems - Fact Sheet / FAQs

This page last reviewed October 28, 2015

What is OBD II?
    OBD II is an acronym for On-Board Diagnostics II, the second generation of on-board self-diagnostic equipment requirements for light- and medium-duty California vehicles. On-board diagnostic capabilities are incorporated into the hardware and software of a vehicle's on-board computer to monitor virtually every component that can affect emission performance. Each component is checked by a diagnostic routine to verify that it is functioning properly. If a problem or malfunction is detected, the OBD II system illuminates a warning light on the vehicle instrument panel to alert the driver. This warning light will typically display the phrase "Check Engine" or "Service Engine Soon," and will often include an engine symbol. The system will also store important information about any detected malfunction so that a repair technician can accurately find and fix the problem.

Check Engine Lights

What was OBD I?
    On-Board Diagnostics I (OBD I) was California's first OBD regulation which required manufacturers to monitor some of the emission control components on vehicles. Required starting with the 1988 model year, OBD I systems were not fully effective because they were limited to monitoring only a few of the emission-related components, and the monitors were not calibrated to a specific level of emission performance. OBD II was developed to address these shortcomings and make the system more powerful and user-friendly for service technicians.

Why is OBD II needed?
    Even though new vehicles sold in California are the cleanest in the world, the millions of cars on the road and the ever increasing miles they travel each day make them our single greatest source of smog forming emissions. While the new vehicles in California may start out with very low emissions, improper maintenance or faulty components can cause vehicle emission levels to sharply increase. Studies estimate that approximately 50% of the total emissions from late-model vehicles are excess emissions, meaning that they are the result of emission-related malfunctions. OBD II works to ensure that the vehicles remain as clean as possible over their entire life.

Does my car have OBD II?
    All 1996 and newer model year gasoline and alternate fuel passenger cars and trucks are required to have OBD II systems. All 1997 and newer model year diesel fueled passenger cars and trucks are also required to meet the OBD II requirements. Additionally, a small number of 1994 and 1995 model year gasoline vehicles were equipped with OBD II systems. To verify that your vehicle is equipped with OBD II, you can look for the words "OBD II" on the emission control information label attached to the underside of the vehicle hood.

Do other states require OBD II?
    Yes. The United States Environmental Protection Agency (U.S. EPA) requires all 1996 and newer model year passenger cars and trucks sold in any state to meet the U.S. EPA OBD requirements. While U.S. EPA's OBD requirements differ slightly from California's OBD II requirements, systems designed to meet California's requirements are also accepted by U.S. EPA as meeting the federal requirements.  In practice, virtually all vehicles sold in the U.S. are designed and certified to meet California's OBD II requirements, regardless of where in the U.S. they are sold. More information about U.S. EPA's OBD requirements can be found at the U.S. EPA OBD website

What should I do if the warning light comes on?
    Most manufacturers advise having the vehicle serviced as soon as conveniently possible. Since there are many different problems that can cause the light to illuminate, it is hard to generalize how severe a problem may be. However, the problem will often cause a noticeable effect on fuel economy, performance, or the driveability of your vehicle, and extended driving without fixing the problem could possibly lead to damage of other components. Additionally, there are certain malfunctions that can cause the warning light to blink. This indicates that a malfunction is occurring that could be damaging your catalytic converter. Because replacement of the catalyst can be expensive, many manufacturers recommend having the vehicle serviced as soon as possible if the warning light is blinking.

Does the warning light only mean the emissions controls on my car aren't working?
    While all malfunctions that cause the light to illuminate either affect emissions or the ability of the OBD system to work properly, many also can affect fuel economy, and several can cause driveability problems or a decrease in overall performance. Manufacturers generally optimize their vehicles for performance, fuel economy, and emissions. As such, virtually any malfunctioning component can result in the vehicle running in a condition that is less than optimal.

Do I have to go to the dealer to get my OBD II car fixed?
    No. Properly trained and equipped independent shops are capable of utilizing the diagnostic information from the OBD II system and can make repairs just like dealers. In fact, several of the provisions incorporated in the OBD II regulation are intended to make it easier for independent shops to diagnose and repair vehicles accurately and in a cost-effective manner.

    It should be noted, however, that California's emission warranty requires the vehicle manufacturer to repair under warranty any problem that the OBD II system detects if the vehicle is less than 3 years old and has less than 50,000 miles. Manufacturers only authorize their dealers to perform warranty work. Further, components which exceed a defined cost limit at the time the vehicle was produced (currently about $600) are covered for 7 years or 70,000 miles - this list of covered parts, which varies from car to car, should be listed in the owner's manual or accompanying warranty booklet that came with the vehicle.  Additionally, if you have purchased a vehicle that is certified by ARB as a partial zero emission vehicle (PZEV), any problem detected by the OBD II system is covered under warranty as long as the vehicle is less than 15 years old and has less than 150,000 miles. Starting with the 2018 model year, you can purchase a vehicle certified by ARB as a transitional zero emission vehicle (TZEV), which also will be covered under warranty for 15 years or 150,000 miles. The "energy storage device" (i.e., the hybrid battery) on PZEVs and TZEVs are covered under warranty for 10 years. A list of vehicles that are certified as PZEVs and TZEVs can be found at ARB's Drive Clean website (select vehicles with a Smog Rating of 9 to show PZEVs and TZEVs).  

How is Smog Check affected by OBD II?
    In California, technicians are required to perform an OBD II check (visual and functional) during the Smog Check inspection. Specifically, the technician visually checks to make sure the warning light is functional, and then the Smog Check test equipment communicates with the on-board computer for fault information. If a fault is currently causing the light to be on, you need to have the malfunctioning component repaired before you can pass the inspection.

    Additionally, the vehicle stores information known as "readiness indicators" to indicate if the vehicle is ready for an inspection. If too many readiness indicators are "incomplete," the vehicle will fail the inspection because it means that the vehicle has not been operated enough since the on-board memory was last cleared to allow all of the OBD system checks to complete. This can occur if a fault has recently been repaired, or if you have recently had a dead, disconnected, or replaced battery. It does not necessarily mean that anything is wrong with your car - it simply means that the vehicle hasn't had a chance to run all of its self-diagnostics to confirm that everything is okay. The vehicle will need to be driven more before the vehicle can be tested to pass.  Vehicle owners who fail Smog Check due to incomplete readiness indicators should drive their vehicle as they normally do for about a week or so to set these readiness indicators to "complete." If the incomplete readiness indicators were most likely not a result of a recently disconnected/replaced vehicle battery, or if the vehicle is not driven regularly, the vehicle owner may wish to seek repair technician help in setting readiness. The technician can access technical information from the vehicle manufacturer and should be able to advise the owner of specific driving patterns needed to set the indicators, or may be able to operate the vehicle (most likely on a dynamometer in the shop) to set the monitors. The technician will have access to a scan tool to determine which monitors have not completed.

    Smog Check inspections for 2000 and newer model year vehicles are now primarily based on an inspection of the OBD II system; tailpipe testing is no longer required. 1996 through 1999 model year gasoline vehicles receive both an OBD inspection and tailpipe testing. In addition, 2000 through 2007 model year medium-duty vehicles (vehicles with a gross vehicle weight rating between 8,500 and 14,000 lbs.) with federal-only certified OBD systems may require both an OBD inspection and tailpipe test.

    More detailed information about California's current OBD II-based Smog Check program can be found at BAR's Smog Check website.

Does OBD II prevent me from using non-OEM parts or modifying my car?
    No. Aftermarket parts manufacturers continue to produce replacement parts to fit most vehicles. These parts are required to be functionally equivalent to the original equipment manufacturer (OEM) parts and are, therefore, compatible with the vehicle's OBD II system. For add-on or performance enhancing parts, aftermarket manufacturers are required to obtain an exemption from ARB before legally offering such parts for sale in California. The process requires the aftermarket manufacturers to demonstrate that their products are compatible with vehicle OBD II systems.  Parts that have been granted such approval can be found at ARB's aftermarket parts database website. Vehicle owners should make sure add-on and modified powertrain products have a valid Executive Order from ARB that permits their use on California registered on-road vehicles before installing such products.

If I need to replace the catalyst (or catalytic converter) on my OBD II car, can I use any catalyst that is available?
    No.  California has specific regulations defining minimum performance levels for catalysts on all cars, including those that are OBD II-equipped. Replacement catalysts available from the dealer for your specific vehicle are legal. Additionally, aftermarket catalysts that have been approved by ARB are legal for use on cars in California. If you are purchasing a new catalyst for your OBD II vehicle, you need to make sure it is approved by ARB for use on your specific vehicle. Approved aftermarket catalytic converters can be found on ARB's aftermarket parts database website. The aftermarket catalyst manufacturers also have catalogs or online resources that identify which catalysts are approved for specific vehicles in California.

How much do OBD II systems add to the cost of a new car?
    In most cases, equipping a new vehicle with an OBD II system has only required minimal additional hardware, resulting in only slight additional costs. This is because most OBD II requirements can be met by only adding new software in the vehicle's on-board computer. In 1996, the federal government estimated that the OBD II requirements increased the retail cost of a 1996 model year new vehicle by an average of $61. Overall, OBD II is anticipated to result in cost-savings to the consumer by catching faults quickly (before other components can be damaged) and by pinpointing the source of the fault to aid technicians in making fast, effective repairs.

What is OBD III?
    OBD III is a term used to describe the concept of "remote OBD." Under this concept, vehicle would have the ability to transmit OBD fault information to roadside receivers, for example, through cellular networks. The benefit of such a concept is that a motorist would not have to take their vehicle to a station for an emissions inspection as long as the vehicle is communicating that there are no active emission-related malfunctions. If the OBD II system has detected a problem, the vehicle owner would be expected to have the problem repaired in a timely fashion. Correction of the problem would be verified through the OBD data transmitted after the vehicle has been serviced.  

    California has not adopted any kind of mandatory remote OBD program at this time. The remote OBD concept has been studied by states, including California, through pilot programs that are based on voluntary participation. ARB's OBD II regulation does not require manufacturers to equip vehicles with the ability to wirelessly transmit OBD information. Vehicles typically must be retrofitted with equipment that can transmit the OBD data in order to participate. Overall, the potential benefits of the remote OBD concept are added convenience and reduced inspection costs to owners of vehicles that are in proper operating condition and greater emission reductions from the more rapid identification and repair of emission-related malfunctions when they do occur.