OBD-II PIDs

OBD-II PIDs  (On-board diagnostics  Parameter IDs) are codes used to request data from a vehicle, used as a diagnostic tool. SAE standard J/1979 defines many PIDs, but manufacturers also define many more PIDs specific to their vehicles. All light duty vehicles (e.g. less than 8,500 pounds) sold in North America  since 1996, as well as medium duty vehicles (e.g. 8,500-14,000 pounds) beginning in 2005, and heavy duty vehicles (e.g. greater than 14,000 pounds) beginning in 2010, are required to support OBD-II diagnostics, using a standardized  data link connector, and a subset of the SAE J/1979 defined PIDs (or SAE J/1939 as applicable for medium/heavy duty vehicles), primarily for state mandated emissions inspections. 

Typically, an automotive technician will use PIDs with a scan tool connected to 

the vehicle's OBD-II connector. 

 - The technician enters the PID 

 - The scan tool sends it to the vehicle's bus (CAN, VPW, PWM, ISO, KWP. After 2008, CAN only) 

 - A device on the bus recognizes the PID as one it is responsible for, and reports the value for that PID to the bus 

  - The scan tool reads the response, and displays it to the technician

CAN (11-bit) Bus format

The PID query and response occurs on the vehicle's CAN Bus. Standard OBD requests and responses use functional addresses. 

The diagnostic reader initiates a query using CAN ID 0x7DF, which acts as a broadcast address, andaccepts responses from any ID in the range 0x7E8 to 0x7EF. ECUs that can respond to OBD queries listen both tothe functional broadcast ID of 0x7DF and one assigned ID in the range 0x7E0 to 0x7E7. 

Their response has an ID oftheir assigned ID plus 8 e.g. 0x7E8 through 0x7EF. This approach allows up to eight ECUs, each independently responding to OBD queries. 

The diagnostic reader canuse the ID in the ECU response frame to continue communication with a specific ECU. In particular, multi-frame communication requires a response to the specific ECU ID rather than to ID 0x7DF.CAN bus may also be used for communication beyond the standard OBD messages. 

Physical addressing usesparticular CAN IDs for specific modules (e.g., 720 for the instrument cluster in Fords) with proprietary framepayloads.

OBD-PID.pdf

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