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On-board Diagnostics Frequently Asked Questions

This FAQ is designed to answer common questions about On-board Diagnostics systems and how they work.

OBD, or on-board diagnostics, is a computer technology that monitors a vehicle’s emission control system and other key engine parts to ensure they’re working properly. When a vehicle’s on-board computer detects an emission problem, it turns on a dashboard warning light to alert the driver, much like the way the oil light appears when a vehicle is low on oil. The OBD system can detect problems drivers do not.

Newer vehicles operate cleaner due to improved technology and sophisticated emission control
systems, but these systems must be in proper working order for the vehicle emissions to remain low. When an engine is not running as designed, performance is lost, fuel is wasted and air pollution increases. By detecting emissions control deterioration and/or failures, and alerting the driver to the need for attention, vehicles can be properly serviced before more serious problems develop.

The DEQ uses the OBD system to test the following vehicle types:

  • 1996 & newer gasoline powered vehicles up to 8,500 lbs (GVWR)
  • 2013 & newer gasoline powered vehicles greater than 8,500 lbs (GVWR)
  • 1997 & newer diesel powered vehicles up to 8,500 lbs (GVWR)

NOTE: GVWR stands for gross vehicle weight rating. Some older vehicles may have a similar computer system, but the connections and problem codes may be different.

The OBD test is both quicker and more effective in identifying polluting vehicles than the tailpipe test. It provides more specific information about what needs to be repaired than a tailpipe test does. It can also quickly eliminate clean vehicles from needing further action.

The tailpipe test is for 1995 and older vehicles.  Our inspectors place a monitoring probe directly into the tailpipe to measure pollution levels.  If there is excessive pollution, it means the emissions control system is not functioning properly, the vehicle needs repair and it will fail DEQ’s test.  This test includes a visual inspection of the vehicle.

The OBD test is for 1996 and newer model years.  Instead of measuring at the tail end of the vehicle, the OBD system identifies earlier in the pollution control system how each pollution control component (e.g. catalytic converter) is functioning.  The OBD system can determine which components are failing, which means the emissions control system is not functioning properly, the vehicle needs repair and it will fail DEQ’s test.  This test does NOT include a visual inspection.

Many states have chosen not to visually inspect vehicles during OBD tests. These states include, but are not limited to Colorado, Delaware, Indiana, Maryland, Massachusetts, Nevada, North Carolina, Oregon, Rhode Island, Washington and Wisconsin.

Here’s why:

  • Redundancy:  The OBD test is considered the most effective vehicle emissions inspection by Environmental Protection Agency (EPA) and that’s even without a visual inspection which EPA does not require.  The OBD test can determine if a catalytic converter is missing, for example.  A visual inspection is considered redundant of the OBD test and most states cannot justify the cost and time to also conduct a visual inspection.
  • Complexity: The system is comprised of multiple components such as evaporative control parts, oxygen sensors and exhaust-gas-recirculation (EGR) just to name a few.  Systems are becoming so much more complex and each manufacturer has variation in engine compartment configurations.
  • Time, Training & Money:  A lot of time and technical training is necessary for full visual inspections.  States that do visually inspect computerized vehicles typically charge more money for emissions testing and typically combine it with their state mandated safety inspection for brakes, lights and tires (which they also charge for).  Oregon does not have a state mandated safety inspection program and our testing fee is one of the lowest in the nation.
  • False Failures:  Some states indicate that inspectors were accidentally visually failing vehicles that shouldn’t have failed so they discontinued visual inspections.

  • Defeat Devices:  An example would be if a catalytic converter is missing or hollowed out and a defeat device is installed to trick the OBD system into thinking there is a functioning catalytic converter. Our software can catch the majority of issues; even if components are missing except for when defeat devices are used. While our software cannot catch everything, it is capable of determining if some defeat devices are being used such as when vehicles use certain fraudulent OBD “simulators”. It is a criminal violation of the Federal Clean Air Act to alter or tamper with OBD systems. If it becomes evident that a business has been tampering with or altering OBD systems, an investigation will be launched. DEQ has participated in such investigations in the past; including against individual motorists.
  • Oregon did use a visual inspection for a period of time:  The use of defeat devices was very low at .01%.  Such a low, rare use of defeat devices is consistent for other states data.
  • Even the best visual inspection will not catch all cheating:  There are some defeat devices that are very difficult to catch.
  • Oregon’s VIP program protects air quality more than most states:  Oregon requires each and every failing vehicle to be fully repaired before allowing the vehicle to pass the test.  That’s not the case in almost every other state where they allow a repair waiver for any motorist spending at least $300 on repairs (some are as low as $150 some are higher).  After failing a vehicle emissions test in most other states, if the motorist fails a second time but provides their repair receipt, the state waives the requirement to pass the emissions test and allows the failing vehicle back on the road to continue spewing excessive air pollution.  While Oregon does provide financial assistance to help with repairs, we require full repair which is more protective of air quality than most states.

We did do visual inspections on a case-by-case basis for a period of time already and found a low rate of defeat devices used (.01%).  The agency periodically reviews the Vehicle Inspection Program as part of the State Implementation Planning (SIP) process.  The last review was in 2006 and it’s being evaluated again during 2017.  If, as part of that process, there is a concern that we will not continue meeting the ozone standard into the future, we will look at all options on how VIP can be improved.  That could include an idea we’ve had to visibly inspect a truly random subset of OBD vehicles chosen by the software.  That could even be employed in the DEQ Too program (remote OBD) whereby random vehicles using that option would be required to be visually inspected at a Clean Air Station.

You aren’t required to do anything before the OBD test. You may check your vehicle’s OBD connector to see if it is damaged or missing. The connector is located in different places for different vehicles. Usually, it is in front of the driver underneath the dash. On a few vehicles, it is hidden behind the ashtray or some other out of-sight location. Your vehicle’s owner’s manual may have more information. If your vehicles check engine light is illuminated the OBD system has detected an emissions related malfunction and will not pass the emissions test.

  • Readiness: 
    To help ensure the OBD system is working properly, “readiness status” is used to indicate whether or not monitored emissions control systems have been tested by the OBD system. Each emissions control system has its own monitor and related readiness status. If any of the readiness status’ are set to "not ready" the OBD system has not yet completed testing of that particular component or system. A component failure may exist, but has not yet been identified because the system testing has not been completed. 
  • Unready: 
    Before an OBD test can be done, most of the emissions control system monitors have to complete their checks of the emissions control equipment. Depending on the age of your vehicle, if one or two monitors have not yet completed their checks, the vehicle is “not ready” for an OBD test. If your vehicle is Unready, you will need to drive your vehicle further to allow these monitors to finish testing the equipment. The amount and type of driving needed varies according to the vehicle. You may need to check with your dealer or repair facility for the specific drive cycle for your vehicle. It’s always a good idea to test your vehicle as early as possible after receiving your renewal notice, in case of a delay such as this. An emission test is valid for 180 days, so you can have your test done whenever it is most convenient for you.
  • Drive Cycles: 
    A drive cycle is a manner of driving that enables the OBD computer to run all emission control systems monitors. There is no set amount of mileage needed to complete a drive cycle. The drive cycle varies in length and includes highway, stop and go, and letting the engine cool completely between trips.

​This is a warning light located on the vehicle's dashboard. The color of an illuminated check engine light can be red, amber, or yellow. If the check engine light stays illuminated while the engine is running, the vehicle's OBD system has detected a potential emissions problem. The check engine light can read, "Check Engine," "Service Engine Soon," or simply be an image of an engine.

Check Engine When the OBD system finds a fault in an emission control system it will illuminate the check engine light letting you know that you need to have your vehicle serviced.  This comes on when a failure occurs which could cause vehicle emissions to exceed designed standard. The check engine light also illuminates when a problem is detected in a component that is used as part of the diagnostic strategy for any other monitored system or component. The purpose of the check engine light is to alert the driver to the malfunction so service can be performed in a timely manner. If your check engine light comes on take your vehicle to a repair technician with the appropriate equipment to diagnose and repair OBD systems. The DEQ keeps a list of recognized auto repair shops. If the check engine light is flashing, vehicle operation is discouraged until the malfunction is repaired. Refer to your vehicle owner’s manual for specific information about your vehicle’s check engine light.

Diagnostic trouble codes are alpha numeric codes that are assigned to specific vehicle malfunctions. Most DTCs have definitions that can provide valuable repair information. EX: P0420 Catalyst System Efficiency Below Threshold (Bank 1).

​A flashing check engine light can indicate a diagnostic trouble code that has the potential to inflict significant damage to other emissions control devices, potentially increasing the cost of future repairs. 

Most OBD vehicles complete a series of manufacturer defined diagnostic checks for evaporative gas leaks. A broken gas cap, or a gas cap not completely tightened, can cause a check engine light. In these cases, the vehicle's OBD system is operating as designed, as it has detected a fault condition. If you believe the gas cap was not completely fastened, simply re-tighten the cap. The vehicle will continue to run its diagnostics even with an illuminated check engine light. The vehicle's OBD system is also capable of extinguishing the check engine light if the fault condition no longer exists after several "trips." Otherwise, you should have your vehicle evaluated, or "scanned," to identify the diagnostic trouble code (DTC) that caused the check engine light to be illuminated. Continued operation of a vehicle with an illuminated check engine light could result in significant damage to vehicle's emission control system and potentially more expensive repair costs.

Usually nothing. If the condition that initially caused the check engine light illumination has been addressed, the vehicle's OBD system is capable of turning the check engine light off. This sequence of check engine light on / check engine light off does not indicate a defective OBD system. The vehicle may have detected a problem that has been corrected without a formal repair (perhaps a loose cap was tightened). Your vehicle needs no special attention unless the check engine light comes on again.

​If your vehicle fails the emission test, it will need to have repairs done before it can be retested. The test station staff will give you a list of recognized auto repair shops. All the shops on this list are able to diagnose and repair problems identified by OBD systems. No fee will be charged.

​Only the OBD computer itself and the catalytic converter are warranted for eight years or 80,000 miles. However, many automakers provide extended warranty coverage beyond that required by law. Check your vehicle’s warranty documents for more information.

​Anyone can work on your vehicle, but it’s recommended that a recognized auto repair shop be used. The DEQ keeps a list of recognized auto repair shops.

​Today's vehicles are highly sophisticated. While OBD helps to ensure that these vehicles are running properly, you still need to maintain your vehicle according to the manufacturer's recommended maintenance and service schedule. Keep up with routine maintenance and keep an eye out for illumination of the Check Engine light.

Lastly, drive as little as possible by combining trips, carpooling, walking, biking, or using public transportation as these all minimize pollution from motor vehicles.