OBD-II J1979 Protocols

On-board diagnostics (OBD) is an automotive term referring to self-diagnostic and reporting capability of a vehicle. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems.The earlier OBD interfaces are OBD1,OBD1.5 etc.Now, Most of the vehicles have OBDII interface, which is an enhanced diagnostic monitor. Refer Wiki OBD page for details.

OBD-II provides access to data from the engine control unit (ECU) and offers a valuable source of information when troubleshooting problems inside a vehicle. The SAE J1979 standard defines a method for requesting various diagnostic data and a list of standard parameters that might be available from the ECU. The various parameters that are available are addressed by "parameter identification numbers" (parameter IDs or PIDs) which are defined in J1979. Refer Wiki PID page for details.

Any vehicle manufactured after 1996 required to have OBD-II computer system. The system can be access through a 16 pin Data Link Connector (DLC). In cars the port is generally located under the dash near the driver; such that is easily accessible. 

OBD-II Protocols

There are five different protocol standard defined under OBD-II. We will here discuss about the data format of each of the protocols in both Single and Multi frame format. OBD supports ELM327 compatible commands. In general OBD command has the format of <OBD mode><PID>\r. For example, In order to query engine RPM from the vehicle, the incoming command to ECU should be 010D\r. Where, 01 is OBD mode and 0D defines the PID for RPM.The response would vary depending on the underneath protocol of the vehicle. The response data length of one frame is 8 bytes. More than this, the ECU reports as multi frame packet. For example 0902 (VIN) request is a multi frame response.

SAE J1850 PWM This signal is Pulse Width Modulation, which runs at 41.6 kbps. This protocol is generally used on Ford vehicles.
Single Frame (010D)

Message Type	Header	Data Bytes
Request	616AF1	01(Mode)0D(PID)
Response	416B10 (10 is ECU  physical address)	41 (Mode 01 response) 0D(PID) FF (Data)

Multi Frame (0902)

Request

Header ID	Mode	PID
616AF1	09	02

Response

Header ID	Mode	PID	Frame No	Data Bytes	CRC
416B10	49	02	01	00 00 00	31
416B10	49	02	02	47 31 4A	43
416B10	49	02	03	35 34 34	34
416B10	49	02	04	52 37 32	35
416B10	49	02	05	32 33 36	37

SAE J1850 VPW

This protocol is Variable Pulse Width, which runs at 10.4 kbps. GM vehicles typically use this version. Both Single and Multi frame responses resemble as like SAE J1850 PWM protocol.

ISO 15765 CAN

This protocol has been mandated in all vehicles sold in the US from 2008 and later. It’s a two-wire communication method and can run at up to 1Mbps. It has four different variants- 

• ISO 15765 11bit/250Kbps
• ISO 15765 11bit/500 Kbps
• ISO 15765 29bit/250Kbps
• ISO 15765 29bit/500Kbps

ISO 15765 defines four frame types – Single frame, First frame,Consecutive frame and Flow control frame.The Multi-frame contains one First frame and one or more consecutive frames.

Single Frame : For unsegmented messages which can fit into single CAN frame.

Example 010C

The data size of ISO 16765 frame are always of 8-bytes. Byte 0 indicates valid bytes count. Remaining bytes will be padded with 0. Here in the above example next four bytes are the actual data length.

MultiFrame: The first frame of segmented frames follows by consecutive frames.

Example (0902)

Here at D0, 10 means FF, 21 means CF with Sequence number 1 and so on. At D1 14 means length of the valid data to be considered.The above example given for 11 bit can ID. For 29-bit CAN the message structure remains same.

ISO 14230-4 KWP

Keyword protocol runs at 10.2 kbps. It is available in 2003+ vehicles. The header is of 3-bytes in length.

Single Frame (010C)

Rx: C233F1 01 0C 

Tx: 84F110 41 0C 00 30 

MultiFrame (0902)

Rx: C233F1 09 02 

Tx: 87F110 49 02 01 00 00 00 31 

Tx: 87F110 49 02 02 47 31 4A 43 

Tx: 87F110 49 02 03 35 34 34 34 

Tx: 87F110 49 02 04 52 37 32 35 

Tx: 87F110 49 02 05 32 33 36 37 

The header format remains same as single frame.

Request

Header ID	Mode	PID
C233F1	09	02

Response

Header ID	Mode	PID	Frame No	Data Bytes	CRC
87F110	49	02	01	00 00 00	31
87F110	49	02	02	47 31 4A	43
87F110	49	02	03	35 34 34	34
87F110	49	02	04	52 37 32	35
87F110	49	02	05	32 33 36	37

In the next chapter we will discuss about DTC.