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OBD2 Standard Tech Info
OBD2 Technical>Standard OBD2
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. Equipment and Tool Institute has a nice
interface by manufacturer page.
Scantool.net also has a nice page listing
vehicles and OBD2 protocols, and
another describing a method of determining interface
by connector pin layout.
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
OBD2 Header Bytes
There are 3 bytes in a standard (omits KWP2000 and CAN) OBD2 header. These bytes
control message priority, describe the request addressing method, and the target
that responses should be directed to.
Example 1: 68 6A F1 (functional mode, default priority and destination)
- 68 is the option byte, bits from most significant to least significant, value - descr:
011(3) - first 3 bits are bus priority, (lower numbers are higher priority)
0 - always 0, have seen references that this bit involved with header byte length
0(disallowed) - bit describes inframe responses required or disallowed
0(functional) - bit describes address mode, 0 is functional, 1 is physical
00(function) - last 2 bits describe msg type, functional 0=function,1=broadcast,2,3???
- 6A is the functional code for request info, would be the physical address in physical mode
(note that answers in functional mode return 6B in 2nd byte, hidden from ELM users)
- F1 is the default scantool address
Example 2: 6C 10 F1 (physical addressing mode, default priority and destination)
- 6C is same as 6A above with 6th bit set for physical addressing
- 10 is the engine controller, a list of controller ranges is available from SAE (or with use of a search engine)
- F1 is the external scantool address
The ELM32x series converter chips do not require that the header bytes be present in messages sent. They will
automatically be generated at the converter. ELM32x v2.0 chips allow the header bytes to be changed with the command
ATSH h1h2h3.
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