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Application Note: On Board Diagnostics

On-Board Diagnostics, in an automotive context, is a generic term referring to hardware diagnostic capability that is present in most cars and light trucks on the road today. During the 1970s and early 1980s manufacturers started using electronic means to control engine functions and diagnose engine problems. This was primarily to meet EPA emission standards. Through the years on-board diagnostic systems have become more sophisticated.

On-Board Diagnostic Systems
  • OBD-II
  • EOBD and JOBD
  • CAN Bus

OBD-II

OBD-II, a standard introduced in the mid-1990s, provides almost complete engine control and also monitors parts of the chassis, body and accessory devices, as well as the diagnostic control network of the car. OBD-II is mandated by the California Air Resources Board (CARB). Vehicle manufacturers in the United States were required to switch their vehicles' diagnostic ports to OBD-II in 1996 ; many 1994 year models had proprietary OBD connections.

OBD-II is primarily a read-only interface for obtaining DTCs (Diagnostic Trouble Codes). OBD-II also defines several commands, such as output state control, self test modes, and KAM (Keep Alive Memory) reset.

The OBD-II connector is identical in all models, but three different protocols are commonly used to communicate. These are:

  • VPW (Variable Pulse Width): General Motors
  • PWM ( Pulse-Width Modulation ): Ford
  • ISO 9141: Chrysler , Asian, and European

OBD-II provides information such as:

  • Speed
  • RPM
  • Oxygen sensor
  • Error codes

EOBD and JOBD

In Europe the EOBD (European On-Board Diagnostics) system was mandated by European Directive 98/69/EC for all petrol vehicles made from 1 January 2001. It is similar to the American OBD-II standard. In Japan, the JOBD system is used.

CAN bus

Some newer cars (usually post-2003) also support the CAN bus, a local area network in some cars that also gives access to additional hardware like radios, navigation systems, and more. By 2008, all vehicles sold in the US must come equipped with CAN bus. CAN bus devices are usually connected to via the OBD-II connector.

Data Loggers for OBD-II

 

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