Termination Resistor

📖 YOUCANIC Automotive Glossary

A termination resistor is a 120-ohm resistor placed at each end of a CAN Bus network to prevent electrical signal reflections that would corrupt data communication. CAN Bus operates as a transmission line — without proper termination, signals reaching the end of the wire reflect back and interfere with new signals being transmitted, causing communication errors. Each CAN Bus network requires exactly two 120-ohm termination resistors, one at each physical end of the bus. These resistors are typically integrated into the control modules located at the two endpoints of the CAN wiring (often the ECU at one end and the instrument cluster or gateway at the other).

DIYers can quickly check CAN Bus termination by measuring resistance between CAN High (pin 6) and CAN Low (pin 14) at the OBD2 connector with the ignition off. The expected reading is approximately 60 ohms (two 120-ohm resistors in parallel). A reading of 120 ohms means one termination resistor is missing (one endpoint module is disconnected or has failed). A reading near 0 ohms indicates a short between CAN High and CAN Low. An open circuit (infinite resistance) means both terminations are missing or the CAN wiring is completely broken. This simple 10-second resistance check at the OBD2 port is the first diagnostic step for any CAN Bus communication problem.