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

This page last reviewed April 07, 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 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." The system will also store important information about the 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 on all 1991 and newer vehicles, OBD I systems were not as effective as possible because they were limited to monitoring only a few of the emission-related components and they were not calibrated to a specific level of emission performance. OBD II was developed to address these shortcomings and make the system more 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 the vehicle emission levels to sharply increase. Studies estimate that approximately 50% of the total emissions from late-model vehicles 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 gasoline and alternate fuel passenger cars and trucks are required to have OBD II systems. All 1997 and newer diesel fueled passenger cars and trucks are also required to meet the OBD 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 (EPA) requires all 1996 and newer vehicles sold in any state to meet the EPA OBD requirements. While EPA's OBD requirements currently are slightly different from California's OBD II requirements, systems designed to meet California's requirements are also accepted by 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.

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, often the problem will have a noticeable effect on fuel economy, performance, or driveability of your vehicle, and extended driving without fixing the problem could possibly even damage other components. Additionally, there are certain malfunctions that can cause the warning light to blink. This indicates that a malfunction is currently occurring which 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 do affect emissions, 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 three years old and has less than 50,000 miles and only dealers are allowed to perform warranty work. Further, major emission components which exceed a defined cost limit at the time the vehicle was produced (currently about $500) 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.  A list of vehicles that are certified as PZEVs can be found at ARB's Drive Clean website (select vehicles with a Smog Score of 9 to show PZEVs).  

How is Smog Check affected by OBD II?

    In all areas of the state, technicians are required to perform an OBD II check (visual and functional) during the Smog Check inspection. Specifically, the technician visually checks for a functioning and an illuminated warning light and 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 test.

    Additionally, if too many readiness flags are "incomplete," the vehicle will fail the inspection because it has not been operated enough to allow all of the self-diagnostics to run. This can occur if a fault has recently been repaired, if you have recently had a dead or disconnected battery, or if your vehicle battery has recently been replaced.  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 flags should drive their vehicle as they normally do for about a week or so to set these readiness flags to "complete."  If the incomplete readiness flags were most likely not a result of a recently disconnected/replaced vehicle battery, or if the vehicle owner does not normally drive the vehicle that often, then the vehicle owner should seek technician help in setting the flags.  The technician should either advise the owner of specific driving patterns needed to set the flags or operate the vehicle himself (most likely on a dynamometer in the shop) and check with his scan tool to determine which monitors have completed.  In the future, OBD II-equipped vehicles may not even have to undergo a tailpipe test. Technicians would simply be required to perform the OBD inspection. 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 modifying (or using non-OEM parts on) my car?

    No. As in the past, aftermarket parts manufacturers continue to produce parts to fit most vehicles. Despite OBD II, the vast majority of parts on a new vehicle are functionally identical or very similar to pre-OBD II equipped vehicles. For add-on or performance enhancing parts, aftermarket manufacturers are still required to obtain an exemption from the Air Resources Board before being legal for sale in California. Already, performance parts have been approved for many OBD II equipped vehicles and they should be available in the future as well.  Parts that have been granted such approval can be found at ARB's aftermarket parts database website.

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 OBD II-equipped cars.  Replacement catalysts available from the dealer for your specific vehicle are legal.  Additionally, aftermarket catalysts which 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 by going to ARB's aftermarket parts database website and selecting "OBD II catalytic converter" as the device type under the "List Executive Orders by Device" section.  The aftermarket catalyst manufacturers also have catalogs and sometimes websites 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 are usually met by 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?

    First, there is no such thing.  There has been speculation about a new OBD program that would utilize remote transponders (like those currently used for automated bridges or toll roads) to send information indicating if any malfunctioning component is present in the vehicle in lieu of having the vehicle inspected at a Smog Check facility every one or two years.  Many have referred to such a concept as OBD III.  However, contrary to the rumors, no such program has been adopted by ARB nor have any decisions been made by ARB to pursue such an approach in California.  The concept certainly exists and there are various products consumers can buy to remotely monitor their vehicle.  Some other states are even pursuing pilot programs and allowing consumers who voluntarily equip their vehicles with such devices to be exempted from their inspection programs that are similar to California's Smog Check.  These other states have pursued such approaches as additional ways to reduce consumer inconvenience and costs of participating in the inspection program.