CAN Bus Diagnostics for Beginners: A Practical Introduction

CAN high and CAN low are a twisted pair terminated at both ends with 120-ohm resistors. Measure resistance across the two wires with the key off and every module powered down: 60 ohms is healthy (two 120s in parallel), 120 ohms means one terminator is open, and infinite means both are gone or the pair is broken. This is a 30-second test that identifies half of all bus problems.

Every module on the bus sees every message. Each message has an identifier that determines its priority and content. A module either responds to a message or ignores it. When two modules transmit at the same time, the lower numerical ID wins arbitration — that is why safety-critical modules use low IDs.

A single failed module can flood the bus with error frames and take every other module offline. Symptom: multiple U-codes across the vehicle, all modules reporting lost communication with each other. Disconnect modules one at a time until the bus recovers, and the one you disconnected last is the culprit.