CAN bus
Workshop glossary › Diagnostics & protocols · UpdatedController Area Network — the two-wire serial bus that ties together every ECU in a modern car. Standardised as ISO 11898.
CAN bus (Controller Area Network) is the differential two-wire serial bus that almost every ECU in a modern car uses to talk to every other ECU. Bosch designed it in 1986 and it was standardised as ISO 11898 in 1993. By 2008 every OBD-II vehicle sold in the US was required to use CAN for its diagnostic link.
The two wires are called CAN-H and CAN-L. At rest both sit at about 2.5 V; when a node transmits a dominant bit, CAN-H rises to ~3.5 V and CAN-L drops to ~1.5 V, giving a differential of ~2 V. The bus is terminated with 120 Ω resistors at each end — measuring 60 Ω across the bus with the ignition off is the quickest way to confirm both terminators are present.
Most cars run multiple CAN networks at different speeds: a high-speed powertrain CAN at 500 kbit/s, a body/comfort CAN at 125 kbit/s, and an infotainment bus that may be CAN, MOST, or Ethernet. The gateway module bridges them and is what the diagnostic connector usually talks to.