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Onboard Diagnostics

Q: What is Onboard diagnostics?

A: Onboard Diagnostics, or "OBD II," is a computer-based system required in the design of all model year (MY) 1996 and newer light-duty cars and trucks. OBD II monitors the performance of the vehicle's power train (engine, transmission) and emission control systems. If the OBD II system detects a problem that could cause elevated emissions, it alerts the driver by illuminating the malfunction indicator lamp ("MIL") or "Check Engine" light. OBD II also assists qualified repair technicians identify and repair problems by storing a "trouble code" and "freeze frame" data for the encountered malfunction.

Earlier versions of OBD were introduced in the early 1980s as electronic systems replaced mechanical systems. The federal 1990 Clean Air Act Amendments required vehicle manufacturers to install OBD II.

Important OBD II Facts

  • If your car or truck was manufactured in 1996 or later, it has a built-in OBD II computer system that monitors the engine and emissions control system.
  • The onboard computer turns on the "Check Engine" light if it finds a problem with your vehicle's operation.
  • If your "Check Engine" light comes on, you should have your car serviced by a professional technician as soon as possible.
    New York State requires annual OBDII emissions inspections for applicable non-diesel and non-electric vehicles under the New York Vehicle Inspection Program ("NYVIP"). Model year 1997 and newer light-duty diesel vehicles will be subject to NYVIP OBD II inspections beginning on January 1, 2012. The NYVIP OBDII inspection requires the connection of approved test equipment to the vehicle's standardized OBD connector to download emissions systems' information.
  • OBD II technology can prevent costly repairs by flagging problems before they become major. Responding to the "Check Engine" light in a timely fashion can improve driving ability and fuel efficiency, enhance your vehicle's performance, and protect the environment by reducing vehicle emissions.
  • While newer OBD II vehicles are designed and manufactured to meet cleaner emissions standards, their emissions will only remain low if these vehicles are properly maintained. OBD II aids automotive technicians in finding and repairing emission-related problems.
    By detecting emissions control component deterioration or failures, vehicles can be properly serviced before more serious and more expensive problems develop. OBD II actually helps repair technicians do their jobs more quickly and reliably, helping you to avoid unnecessary repairs and return trips back to the repair shop.

Q: What is the malfunction indicator light (MIL)?

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

Sample of a check engine light - the shape of an engine Another sample of a check engine light. Sample of a service engine soon light

Certain severe engine malfunctions may cause the MIL to blink or flash on and off. These conditions require a reduction in speed and immediate service. Consult your owner's manual for further guidance.

Q: What should I do if the MIL goes out before any repairs are made?

A: Usually nothing. If the condition that initially caused MIL illumination has been addressed, the vehicle's OBD II system is capable of turning the MIL off. This sequence of MIL on / MIL off does not indicate a defective OBD II 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 MIL comes on again.

Q: What are the advantages of OBDII inspections?

A: OBD II offers several advantages over traditional tailpipe-based emissions inspections. OBD II alerts the driver, by illuminating the MIL, of an engine management or emissions control issue once it's been encountered. There are circumstances where the OBD II system may detect a problem before the driver notices an operational issue. Early diagnosis and timely repairs can prevent additional and even more expensive repairs. OBD II also provides the repair technician with information (diagnostic fault codes, freeze frame information) specific to the emissions fault condition that led to MIL illumination. This information allows for a more focused and potentially faster repair.
Whenever an engine is not running as efficiently as possible, performance can be lost, fuel can be wasted, and air emissions are likely to increase. OBD II repairs can result in substantial fuel savings.

OBD II inspections take less time to complete than traditional tailpipe based inspections, and are capable of evaluating evaporative emissions problems (i.e., leaks from hoses) that are not possible for older, pre-OBD II vehicles.

Q: What takes place during a NYVIP OBD II inspection?

A: The complete NYVIP OBD II inspection includes three components:

  • Safety inspection;
  • Visual inspection of the emission control devices (including the gas cap); and
  • OBD II inspection.

The OBD II inspection involves two visual inspections followed by the retrieval of electronic data from the vehicle's on-board computer. The visual inspections are restricted to the operation of the MIL. Electronic OBD II data is retrieved by attaching the NYVIP inspection equipment to a standard diagnostic link connector located inside the vehicle. The OBD II pass/fail determination is based on both the visual inspections and data retrieved from the vehicle.

Q: What should I do if the MIL stays lit?

A: If the MIL remains lit while the engine is running, the vehicle's on-board computer has detected a potential emissions related fault that needs to be evaluated. The problem may increase air pollution, reduce fuel efficiency, or even harm the engine. You should take the vehicle to a qualified automotive repair technician to determine the severity of the problem as soon as a possible. Delaying repairs could result in damage to the vehicle's emission control system that could result in significantly higher repair costs.

A vehicle presented for an official NYVIP OBD II inspection with an illuminated MIL will fail the emissions inspection. Do not wait until the end of your current inspection sticker to have your vehicle inspected. Your repair technician needs adequate time to diagnose and repair the vehicle.

Q: How do I know if my vehicle passed or failed the NYVIP OBDII inspection?

A: The NYVIP test equipment will print a document referred to as the Vehicle Inspection Receipt, or "VIR," that details the results of all components of the inspection. You can request this document from the inspector.

If your vehicle passes the NYS inspection, you will receive an inspection certificate (sticker) that is affixed to the vehicle's windshield.

If your vehicle fails the OBDII inspection, the VIR will list the reason(s) for the failure. The report can also provide useful information to a qualified automotive repair technician should your vehicle require repair.

Q: What needs to happen if my vehicle fails the OBDII inspection?

A: If your vehicle fails the OBD II inspection, the vehicle must pass a re-inspection or receive an emissions-related waiver (see below) in order to receive a valid inspection certificate (sticker). In most cases, the vehicle will need to be repaired. In the specific case of a "readiness failure" (see below), the vehicle must be driven to allow the vehicle's OBD II to complete an adequate number of diagnostic checks to become "ready."

Failure to pass the required NYS inspection by the expiration of the current inspection certificate will leave the motorist subject to fines (i.e., expired inspection sticker) and possibly registration denial.

Q: My vehicle failed the NYVIP OBD II inspection for the readiness evaluation. What does this mean, and how can I get my vehicle to pass?

A: A readiness failure means that the NYVIP inspection equipment communicated with the vehicle, and the vehicle reported that it had not completed a sufficient number of its supported on-board diagnostics. The vehicle will need to be driven for a sufficient number of diagnostics to run to completion. The following guidance applies specifically to the OBD II readiness evaluation:

  1. Motorists are encouraged to have their vehicle diagnosed when the MIL is illuminated. Do not disconnect the battery in an attempt to bypass a NYVIP OBDII inspection. This practice is counterproductive, as it will lead to a NYVIP readiness evaluation failure.
  2. Motorists are encouraged to have their annual inspection completed prior to the last week of their valid inspection sticker. Some vehicles are more difficult to make "Ready" than others, and multiple attempts at completing a recognized drive cycle may be necessary.
  3. Motorists receiving a 10-day extension should be cautioned that this extension is for one time only. The inspector or motorist should drive the vehicle for several days in an attempt to set monitors, and then have the vehicle re-inspected.

Additional information on readiness failures can be found at

Q: Can a loose gas cap cause the MIL to turn on?

A: Most OBD II 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 MIL illumination. In these cases, the vehicle's OBDII 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 MIL. The vehicle's OBD II system is also capable of extinguishing the MIL 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 MIL to be illuminated. Continued operation of a vehicle with an illuminated MIL could result in significant damage to vehicle's emission control system and potentially more expensive repair costs.

Q: Does my vehicle qualify for an emissions related waiver?

A: Vehicles subject to the NYVIP OBDII inspection may qualify for a waiver if all of the following conditions are met:

  • the vehicle fails only the OBD II emissions test portion of the NYS inspection (i.e., the vehicle must pass the safety, gas cap check, and emission control device (ECD) visual checks);
  • the vehicle receives repairs related to the reason(s) for the OBD II emissions failure;
  • the vehicle fails the OBDII inspection at least twice during its current inspection cycle (i.e., an initial inspection and at least 1 re-inspection); and
  • any qualifying repairs must be documented, and must total at least $450.

Q: Could emissions-related repair costs be covered under the vehicle manufacturer's warranty?

A: Potentially, yes. Warranty coverage varies depending on what components require repair, vehicle age, mileage, and the individual vehicle manufacturer's warranty provisions. It is better to respond to an illuminated MIL sooner than later to minimize the owner's repair liability.

The federal Clean Air Act requires an 8-year or 80,000 mile warranty on the major emissions control components, such as the catalytic converter, and a 2-year or 24,000 mile warranty on other emissions control components. Owners are advised to read the warranty provisions within their owner's manual or warranty booklet.

Q: What else can I do to make sure that my vehicle is running properly, and how can I minimize its environmental impact?

A: Today's vehicles are highly sophisticated. While OBD II 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.