CAN Bus Architecture and Communication Faults
Modern vehicles contain dozens of electronic control modules (ECMs) — engine control, transmission control, body control, ABS, airbag, instrument cluster, HVAC, and more. Rather than each module having its own dedicated wiring to every other module (which would require hundreds of individual wires), the Controller Area Network (CAN bus) protocol allows all modules to share a common two-wire communication network. The CAN bus uses differential signaling on a twisted pair: CAN High (CANH) and CAN Low (CANL). At rest (recessive state), both wires are at 2.5V. When a module transmits (dominant state), CANH rises to approximately 3.5V while CANL drops to 1.5V — the 2V differential is the data signal. Multiple modules can be on the same two wires simultaneously because data is framed in time-multiplexed packets, each with a unique ID header identifying the source and priority. CAN bus faults generate U-code DTCs (U = network communication). Common fault patterns: U0100 (Lost Communication with ECM/PCM) suggests the ECM cannot be reached on the network — possible causes include a faulty ECM, wiring fault in the CAN circuit, or missing terminating resistors. The CAN bus requires a 120-ohm terminating resistor at each end of the bus (two resistors total) to prevent signal reflections. Disconnecting the battery and measuring resistance between CANH and CANL: a healthy bus measures approximately 60 ohms (two 120-ohm resistors in parallel). A reading of 120 ohms means one terminator is missing; infinite resistance means both terminators are missing or the bus wiring is broken; near-zero resistance means a short to ground or between the two wires.