Science Fair Project Encyclopedia
OBD-II (OnBoard Diagnostics version 2) is a common hardware diagnostics interface that is present on all cars sold in the United States after 1996. It is present in some earlier vehicles. OBD-II is mandated by CARB, the California Air Resource Board.
OBD-II is primarily a read-only interface for obtaining DTCs (Diagnostic Trouble Codes). OBD-II also defines several commands, such as output state control, self test modes, and KAM (Keep Alive Memory) reset.
The OBD-II connector is identical in all models, but three different protocols are commonly used to communicate. These are:
- VPW (General Motors)
- PWM (Ford)
- ISO9141 (Chrysler, Asian, and European)
OBD-II provides information such as:
- Oxygen sensor
- Error codes
For a list of generic OBD-II codes, see the Table of OBD-II Codes.
Some newer cars (usually post 2001) also support the CAN BUS, a local area network in some cars that also gives access to additional hardware like radios, navigation systems, and more. CAN BUS devices are usually connected to via the OBD-II connector.
There are a number of multi-thousand dollar interfaces available to professional mechanics, but hobbyists can also connect their PCs to their cars using inexpensive adapters like the following:
- BR-3 (supports all three protocols in one device)
- ScanTool (very popular interface, protocol specific hardware needed)
- CarChip (an unobtrusive data logging device for recording vehicle activity}
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