OBd2 Standards and Terms

OBD2 standard
OBD2 (or OBDII) defines a communications protocol and a standard connecter to acquire data from passenger cars. It was required by U.S. EPA on all gasoline powered cars and light duty trucks manufactured for the U.S. after 1996 to help monitor/inspect vehicle emissions. Canada required it after 1998, and a European standard (EOBD) is also enforced.

OBD2 electrical interfaces
The OBD2 standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.  Here is a brief summary of the most popular electrical interfaces:

  • ISO15765 (CAN) – Controller Area Network, a new higher speed interface, just now being implemented (This protocol is currently not supported by any ELM32x or by our software)
  • ISO9141-2 used mostly by Chrysler and the European/Asian imports
  • ISO14230, or KWP2000 (Keyword Protocol) shares the electrical portion of ISO9141 standard, but defines it’s own commands. (This protocol is currently not supported by the ELM323 (ISO) or by our software)
  • PWM J1850-41.6 (41.6Khz Pulse Width Modulated) used mostly by Ford (supported by the ELM320 chip)
  • VPW J1850-10.4 (10.4Khz Variable Pulse Width) used mostly by GM vehicles (supported by the ELM322 chip)

OBD2 Acronyms
Some additional terms you may run across in OBD2:

  • CTS – Coolant Temperature Sensor, pretty self-explanatory
  • DLC – Data Link Connecter, the standardized plug in the vehicle, a 16-pin trapezoidal connecter, often found under the driver side dash (U.S. EPA’s allowed DLC location GIF)
  • DTC – Diagnostic Trouble Codes, an alphanumeric code indicating a specific fault reported by the OBD system
  • ECM – Electronic Control Module, a vehicle computer
  • ECU – Electronic Control Unit, a vehicle computer
  • EGR – Exhaust Gas Recirculation, to recirculate unburnt volatiles in exhaust
  • HO2S – Heated Oxygen Sensor, refers to oxygen sensors in exhaust path looking for unburnt oxygen
  • ISO – International Standards Organization
  • KOEO – Key On Engine Off, describes some tests without engine running, but ignition on
  • KOER – Key On Engine Running, describes tests with engine running
  • MAF sensor – Mass Air Flow measures amount of air being drawn into intake
  • MAP sensor – Manifold Absolute Pressure, reads air pressure to adjust timing, fuel/air ratios
  • MIL – Malfunction Indicator Light, indicates a trouble code has been recorded. Also referred to as the Service Engine Soon light, the Check Engine light, idiot light, etc. Usually can only be reset by an OBD2 tool (even if the condition is no longer present), will automatically fail any emissions test if lit.
  • PCM – Programmable Control Module, a vehicle computer that can be reprogrammed/have values modified
  • PCV – Positive Crankcase Ventilation – recirculates unburnt volatiles escaping into crankcase
  • SAE – Society of Automotive Engineers
  • Scantool – an OBD2 tool
  • SIR – Supplemental Inflatable Restraint – airbag
  • Tap Test – Gently striking the ECU housing with fingers with engine running to check for intermittent connections
  • TPS – Throttle Position Sensor, monitors position of the accelerator pedal/throttle linkage
  • WOT – Wide Open Throttle, pedal to the metal

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