CAN Bus and Network Communication

Before CAN bus, every module that needed to communicate with another had its own dedicated pair of wires between them. A vehicle with 30 modules needed hundreds of wires just for inter-module communication. CAN bus — Controller Area Network — solved this by connecting all modules to one shared two-wire network. Two wires doing the work of hundreds.

CAN High and CAN Low are the two network wires. At rest both sit at approximately 2.5 volts. When a module transmits, CAN High rises toward 3.5 volts while CAN Low simultaneously drops toward 1.5 volts. The network reads the voltage difference between the two wires — not the absolute voltage of either one. Any noise that affects one wire affects the other equally — the difference stays the same and the signal stays clean.

At each physical end of the CAN bus is a 120 Ohm terminating resistor that prevents signal reflections. Two 120 Ohm resistors in parallel at the OBD-II DLC read approximately 60 Ohms. Disconnect the battery. Measure resistance between OBD-II pin 6 and pin 14. 60 Ohms — both terminators intact and good. 120 Ohms — one terminator is open or missing. Below 60 Ohms — something is loading the network. Above 120 Ohms — open in the bus wiring itself. This 30-second test tells you the health of the entire network.

Modern vehicles run multiple separate networks simultaneously at different speeds for different purposes. HS-CAN — High Speed CAN — operates at 500 kilobits per second. Used for powertrain and chassis systems that need real-time communication: PCM, TCM, ABS module, stability control. These share data in milliseconds because they directly control vehicle behavior and safety. MS-CAN — Medium Speed CAN — operates at 125 kilobits per second. Used for body systems: BCM, instrument cluster, HVAC module, power accessories. These need regular communication but not millisecond response. LS-CAN — Low Speed Fault Tolerant CAN — operates at 33 kilobits per second. Designed to continue operating even when one of its two wires has a fault — used for non-critical systems where communication must survive a single wire failure. LIN bus — Local Interconnect Network — the simplest of all. A single wire connecting simple function modules to a master module that bridges them to the main network. Used for mirror motors, seat modules, simple switches and actuators that only need basic commands.

A fault on the HS-CAN network drops powertrain and chassis modules but leaves body modules on the MS-CAN completely unaffected. They are separate networks. A fault on HS-CAN that keeps the PCM from communicating does not affect the infotainment system. When you see U codes — network communication fault codes — identify which network the affected modules are on before testing anything. The manufacturer schematic shows which modules are on which network. Address every U code before any B, C, or P code on the same vehicle. A module that cannot communicate sets fault codes across every system it can no longer control, making one network fault look like ten separate failures.