CAN bus

The CAN bus is designed to allow low-cost devices to communicate with each other within a vehicle without a host computer.

Higher layer implementations
(in reverse alphabetical order)

SAE J1939
After 1996, most vehicles have a OBD-II data link connector.

EmCan Protocol
"Some of the goals of the EmCan protocol are:

-- EmCan Protocol Specification
 * Hot plugging.
 * Inferred configuration. Sensors report not merely raw ADC counts, but can be queried to discover the physical type of dimension it measures (length, mass, time, force, frequency, etc.) and a scale factor (calibration factor) that can be used to convert the raw data to standard engineering units (meters, kilograms, seconds, Newtons, Hertz, etc.)
 * Uses standard CAN bus frames or a bi-directional byte stream interface
 * Simple enough to be reasonably implemented in a small microcontroller
 * Interoperable. Any EmCan device can be connected to any EmCan bus without harm to the device or the rest of the system."

"The current document is a work in progress ... If you are interested in this concept and are willing to discuss details of your system, talk to me. ... Having another implementation in progress as the spec is being finalized would be good for spec development and to test out the reference implementation" -- Olin Lathrop