On-board diagnostics (OBD) is an automotive term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems. One of its strengths is it can detect problems long before the driver is able to notice any symptoms. Modern OBD implementations use a standardized digital communications port to provide real-time data in addition to a standardized series of Diagnostic Trouble Codes (DTCs), which allow one to rapidly identify and remedy malfunctions within the vehicle.
Early versions of OBD would simply illuminate a malfunction indicator light if a problem was detected but would not provide any information as to the nature of the problem. The main purpose of OBD-I to be implemented was to make auto manufacturers to design reliable emission control systems. Each manufacturer used their own diagnostic link connector (DLC), DLC location, DTC definitions and procedure to read the DTCs from the vehicle. DTCs from OBD-I cars are often read through the blinking patterns of the 'Check Engine Light' or ‘Malfunction Indicator Light’ (MIL).
By connecting certain pins of the diagnostic connector, the 'Check Engine' light will blink out a two-digit number that corresponds to a specific error condition.
Advantages of OBD-II over OBD-I
OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signaling protocols available, and the messaging format. OBD-II is a sort of computer which monitors emissions, mileage, speed, and other useful data. The OBD-II standard provides extensible lists of DTCs.
OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter:
- P for engine and transmission (powertrain)
- B for body
- C for chassis
- U for network.
All OBD-II cars have a connector located in the passenger compartment easily accessible from the driver's seat. The port, also known as the J1962 diagnostic connector is usually located behind the dashboard, above the brake pedal. A cable is plugged into the OBD-II J1962 connector and connected scan tool. The port allows the scan tool to gain information about the vehicle.
OBD-II: Hardware and Communication protocols
While every system is the same for the most part, the systems may vary slightly. These are known as protocols. These protocols define the ports and their pin configuration for communication.
There are five basic signal protocols in use –
- SAE J1850 PWM (Pulse Width Modulation),
- SAE J1850 VPW (Variable Pulse Width Modulation),
- ISO14230-4 (KWP2000), and
- ISO 15765-4/SAE J2480 (CAN-BUS).
- CAN has become the new standard, all vehicles after 2008 use CAN.
The J1962 provides two standardized interfaces, namely type A and type B. Both are 16 pin (2x8), D-Shaped female connectors. Both connectors have a groove between two rows of pins but there is an interruption in the groove of type B connector. Type A connector is used for vehicles that use 12V supply and Type B connector for those vehicles which use 24V supply.
Pin 2: Bus Positive Line of SAE J1850 PWM and VPW.
Pin 4: Chassis ground.
Pin 5: Signal ground.
Pin 6: CAN-High (ISO 15765-4 and SAE J2280).
Pin 7: K-Line of ISO 9141-2 and ISO 14230-4.
Pin 10: Bus Negative Line of SAE J1850 PWM only(not SAE J1850 VPW).
Pin 14: CAN-Low (ISO 15765-4 and SAE J2280).
Pin 15: L-Line of ISO 9141-2 and ISO 14230-4.
Pin 16: Battery voltage. Type A- 12V and Type B- 24V.
By examining the OBD II connector, we can tell about the protocol:
- J1850 VPW- The connector should have metallic contacts in pins 2, 4, 5, and 16, but not 10.
- ISO 9141-2/KWP2000 - The connector should have metallic contacts in pins4, 5, 7, 15, and
- J1850 PWM- The connector should have metallic contacts in pins 2, 4, 5, 10, and
- CAN- The connector should have metallic contacts in pins4, 5, 6, 14 and
Data coming from OBD-II is of two forms: Diagnostics Trouble Codes and Real-time monitoring. Real time data can be acquired and stored for further processing and analysis.
- Diagnostic Trouble Code
Diagnostic Trouble Codes (DTC) are values that correspond to a particular type of fault in the vehicle. There are two types of codes, Generic and Manufacturer Specific.
Some generic codes are
- P0500- Vehicle Speed Sensor Malfunction.
- P0520- Engine Oil Pressure Sensor/Switch Circuit Malfunction
- C0000- Vehicle Speed Information Circuit Information Malfunction.
- U0164- Lost Communication with HVAC Control Module.
- B0540- Speedometer Circuit.
- Understanding the Codes:
What can it do?
- Use of OBD-II data
What you need to communicate with OBD II port?
For vehicle testing:
- You need an OEM proprietary .dbc file to understand manufacturer specific codes.
- Generic Codes can be understood by any suitable data acquisition system based on its capability. These are open to all and follow SAE J2280 standard.
For general tracking and use:
- Various apps can be found which provide lots of interesting engine and performance information along with some very useful data which though is not advisable to use for vehicle development but is a good measure for driver to assess his driving habits.
OBD-III, which is still a concept, is advancement over OBD-II which will be equipped with telemetry. Using a radio transponder, OBD-III equipped vehicle will be able to send emissions problem directly to the regulating authority. It will report the Vehicle Identification Number (VIN) and the DTC. This can be done via cellular or satellite link as soon as the MIL turns on.
This will help in cost savings as only vehicles with emission problems need to be tested.
Advanced Structures India Pvt Ltd is an independent automotive product development company based out of Bangalore, India with operations in India, China and US. Above is a blog entry from our engineers about On- Board Diagnostics II. We can be contacted on email@example.com for business enquiries and firstname.lastname@example.org for open positions.