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New Hampshire Department of Environmental Services

On-Board Diagnostics (OBD) Systems

Automobiles manufactured since the early 1980s are equipped with "smart" systems, known as on-board diagnostics (OBD). These systems were developed to help repair technicians identify problems associated with the computerized engine systems of modern vehicles. They are made up of various sensors and a computer that communicates its findings to a technician by means of diagnostic trouble codes that are stored in the automobile's computer.

As a result of the federal Clean Air Act Amendments of 1990, newer more advanced OBD systems, known as OBD II, were built into all vehicles manufactured starting in 1996. OBD II systems monitor vehicle conditions and components that are related to vehicle emissions, such as the catalyst in the catalytic converter, engine misfire, the engine coolant temperature, and the oxygen sensor.

How the OBD System Works
OBD II systems provide owners with an early warning of vehicle malfunctions that can increase emissions, often before the driver is aware of them. When the OBD II system detects a problem, a dashboard "Check Engine" or "Service Engine Soon" light (also known as a Malfunction Indicator Light or MIL, for short) is illuminated. A corresponding diagnostic trouble code is stored in the computer's memory documenting which emissions control component is experiencing the problem, and under what driving conditions. When the vehicle is taken in for service, a repair technician retrieves the diagnostic trouble code information from the computer using a computer "scan tool." By using this information, a properly trained technician can accurately diagnose and fix the problem.

By giving drivers this early warning, OBD protects not only the environment, but also consumers by identifying minor problems before they become major repair bills. For example, by identifying a relatively inexpensive repair like the replacement of a malfunctioning oxygen sensor, OBD can save the owner the cost of replacing the catalytic converter later. OBD can also help save fuel and money by making sure gasoline isn't wasted as a result of, for example, a loose gas cap or incomplete combustion.

New Hampshire Vehicle Safety Inspection Testing Requirements
Beginning in 2005, as required by the federal Clean Air Act Amendments of 1990, New Hampshire's annual vehicle safety inspection will include a test of the OBD II system on all 1996 and newer passenger cars and light duty trucks. This new statewide system will enable inspection stations to perform OBD testing in conjunction with vehicle safety inspections and electronically collect and report the results to the NH Division of Motor Vehicles. This fully automated system (called the New Hampshire OBD and Safety Testing Program) will help ensure that all vehicle inspections are performed properly and fairly, reducing paperwork for the inspection stations, as well as reducing the potential for fraud by allowing immediate detection of "rigged" tests.

A typical OBD system check should add less than five minutes to the current inspection time. If a vehicle fails the OBD testing portion of the inspection, the vehicle operator will be issued a report that documents the reason for the failure. Vehicle owners will have 60 days to have the vehicle repaired and retested.

The information in the electronically generated failure report will help the motorist communicate with their repair technician to identify and complete only the necessary repairs, and prevent needless repairs. Vehicle owners should ask their repair facility if the technicians have received proper training, and have access to the necessary equipment to properly service OBD equipped vehicles.

Benefits of OBD
Modern cars are cleaner (i.e., emit less air pollution) due to newer engine management technology and emission control equipment. However, these cars are only cleaner if all emission control systems are operating properly. A properly operating OBD system monitors the engine, power train, emissions systems, and other components that can affect emissions. It not only alerts the driver of any malfunctioning emissions control components that require attention, but ensures that the vehicle engine runs at peak efficiency.

The OBD system and the use of OBD for inspecting vehicle emission control systems offer the following benefits

  • Improved air quality
  • Early detection and repair of minor problems that may prevent more significant, more costly repairs later.
  • Accurate diagnosis that leads to essential, cost-effective repairs.
  • Early vehicle maintenance opportunity, which leads to greater fuel efficiency and reliability, and potentially less engine wear and tear.
  • Short inspection time for the public since the OBD test is part of vehicle safety inspection, minimizing inspection time and maximizing cost-effectiveness.
  • Incentive to car manufacturers to produce more durable engines and emission controls.
  • State-of-the-art evaporative emission detection, eliminating evaporative emissions as well as exhaust emissions.



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NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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