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On-Board Diagnostics (OBD) II - Frequently Asked Questions

Important: If your "check-engine" light is on, your vehicle will automatically fail the test if it is a 1996 model year or newer.

What is OBD II?

On-Board Diagnostics (OBD II) is a complex computer package installed on 1996-and-newer cars and light trucks, and 1997-and-newer diesel vehicles. This sophisticated system serves as an advanced warning to alert vehicle owners and auto technicians of potentially high emissions. OBD II is now being used by auto emissions testing facilities.

What does my Diagnostic Trouble Code (DTC) mean?

Each DTC indicates the type of fault and the area of the OBD and power train/emission control system where the fault is located. When a problem occurs that may cause the vehicles emissions to exceed applicable standards, the OBD system identifies the failure with a specific DTC, illuminates the malfunction indication light (MIL) and records a record of what has occurred. The link provided below contains a reference list of codes associated with OBD, specifically related to emission system repairs.

Link: OBD DTC reference list for emission repairs

Why is Ohio changing the E-Check test?

Federal regulations require Ohio to implement the OBD II test for most 1996-and-newer vehicles in place of the current tailpipe emissions test. The OBD II test will improve the testing process by detecting problems in a vehicle's emission control systems before they cause costly repairs and allow more harmful pollutants into Ohio's air.  

How does OBD II work?

The computer continuously tracks and stores information about the vehicle"s performance. The on-board computer turns on the "check-engine" or "service-engine-soon" light if it finds a problem with the vehicle's emission control system.

What vehicles are tested using the OBD II test?

Most 1996-or-newer gasoline-powered cars and trucks up to 10,000 pounds (and 1997-and-newer diesel-powered vehicles) can be tested using OBD II. If the vehicle's computer is found to be not ready after communicating with the test equipment during the initial test, the owner must drive the vehicle under normal testing conditions and return at a later time to attempt a retest. As of Jan. 1, 2020, the tailpipe emissions test has been discontinued. All 1996 newer model year vehicles will not be offered the tailpipe test as an alternative.

**NOTE: BMW Series 7 owners must provide accessibility to the OBD computer by removing the cover plate prior to inspection. BMW dealers can complete this task. We regret any inconvenience this may cause.

What are the advantages of using the OBD II test?

OBD II identifies the causes for a vehicle to fail its emissions test. By issuing specific diagnostic codes, OBD II pinpoints the problem, allowing repair technicians to make quicker, more effective repairs. This saves both time and money. OBD II is an early warning system that alerts vehicle owners to emissions problems at a stage where repairs are less costly and perhaps even under warranty. OBD II inspections also are quicker than previous tests.  

What does OBD II have to do with clean air?

While we have made significant achievements in improving air quality in Ohio over the last 20 years, air pollution from automobiles remains a challenge. The number of registered vehicles in Ohio has increased 125 percent since 1970, and vehicle miles traveled have steadily increased as well. 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. OBD II can detect problems drivers do not. 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.  

How is OBD II technology used to test for emissions?

OBD II testing uses a hand-held tool that plugs into the vehicle's computer and determines whether the emissions system and components are working properly. The test downloads information stored in the vehicle's computer to identify malfunctioning emission systems or components. Information can be downloaded only to the E-Check computer system. No information can be altered or added to the vehicle's computer.  

How sophisticated is the OBD II system?

OBD II can detect malfunctioning components and systems before more serious failures occur and even before the driver of the vehicle becomes aware of a problem. The OBD II system enables a vehicle owner to make cost-effective repairs before costly damage is done to the vehicle.

What is the difference between the OBD II and the existing treadmill tests?

The treadmill test measures the concentration of gases coming from the vehicle's exhaust pipe. The OBD II test is an electronic test that uses a hand-held reader device plugged into the vehicle's computer through a port mounted under the dash. The vehicle's computer will communicate if there are any components that are deteriorating or failing that may cause the vehicle to exceed allowable emissions limits.  

Does this mean vehicles won't be tested on the dynamometer anymore?

Yes. Vehicles that have an OBD II system that is not "ready" during the initial test will be rejected from testing and asked to return once the OBDII system is responding as ready.

How does the OBD II system notify the driver of a problem?

When an emissions control malfunction is detected, a dashboard light illuminates with the message "check-engine" or "service-engine-soon." If the OBD II system detects a problem, the on-board computer stores a diagnostic trouble code that identifies the malfunction. Using an OBD II scan tool, a repair technician can quickly retrieve the diagnostic codes and make necessary repairs, often before a more serious problem develops.  

My "check-engine" light is on, but my vehicle is running fine. What should I do?

If the "check-engine" light is flashing, take the vehicle to a repair facility as soon as possible to prevent major damage to your vehicle. If the light is not flashing, and you do not notice any changes to the engine's performance, drive the vehicle in a normal manner for one week to see if the problem corrects itself and turns the light off. If the light remains illuminated after a week of normal driving, take the vehicle to a dealership or repair facility to fix the problem. Read your owner's manual for further information on your vehicle's OBD II system. Your vehicle will fail the E-Check test if the "check-engine" light is on.

What does "not ready" mean?

If a battery is disconnected/replaced or a service technician has turned off the dashboard "check-engine" light after most making repairs, this resets the vehicle's emission system components to "not ready." The status remains "not ready" until the vehicle's computer has had adequate time to review the repaired component and run self-diagnostic tests. This happens after the vehicle is driven for a period of time established by the manufacturer.

If the vehicle's emissions system status is "not ready" when it is presented at the E-Check station the vehicle will be rejected from testing and asked to return once the OBDII system is responding as ready.

For initial and subsequent tests, if a dashboard light is on when the vehicle is presented at the E-Check station, the vehicle will fail the test.  

How can the vehicle status be made ready?

When a vehicle is driven through its normal drive cycle, the computer reviews the emission control system and if the vehicle was properly repaired, the system resets itself to ready. A normal drive cycle includes operation at both cruising speeds and in stop-and-go traffic for up to a couple weeks. This process should be followed before bringing the vehicle in to be tested.  

How can the dashboard light be turned off?

After fixing the problems, the service technician may turn off the dashboard light. There are also situations under which the vehicle's OBD II system can turn off the dashboard light automatically if the conditions that caused a problem are no longer present. If the OBD II system evaluates a component or system three consecutive times and no longer detects the initial problem, the dashboard light will turn off automatically. This could happen if the gas cap is not properly tightened after refueling. The OBD II system will detect the vapor leak and turn on the dashboard light. If the cap is properly tightened, the situation will correct itself and the light will turn off.  

Why would a vehicle fail an OBD II test?

  • Malfunctioning catalytic converter
  • Faulty gas cap
  • Fuel system malfunction
  • Incorrect ignition timing
  • Malfunctioning air injection system
  • Misrouted vacuum lines
  • Contaminated engine oil
  • Faulty positive crankcase ventilation (PCV) system or exhaust gas re-circulation (EGR) system

Can anyone service an OBD II-related problem?

E-Check recommends that motorists utilize qualified, trained technicians equipped with the appropriate diagnostic and repair equipment to conduct OBD II-related service. E-Check lane inspectors can provide a list of Ohio E-Check licensed repair facilities.  

If my "check-engine" light is not on, will I automatically pass the OBD II test?

No. A vehicle may fail the OBD II test even when the "check-engine" light is not on.

Why must I take the OBD II test if my "check-engine" light is not on?

The complete test is performed to ensure that the light is working properly and that all required OBD II monitors have tested the emissions systems and components.

How do I know if my vehicle is covered by warranty?

Federal law requires that the emission control systems on 1995-and-newer model year vehicles be warranted for a minimum of two years or 24,000 miles. Warranty coverage for the on-board computer and catalytic converter (only) is extended to eight years or 80,000 miles for these same vehicles. Many automakers provide extended warranty coverage beyond that required by law. Depending on the model year, emission system repairs may be covered by the manufacturer. Consult your vehicle's warranty documents or your auto dealer for more information.

Additional questions about the Ohio E-Check Program?

Call 1-800-CAR-TEST or (614) 644-3059.