How do I check CAN bus resistance at the diagnostic connector?
Short answer: With the battery disconnected, measure between pins 6 and 14 of the OBD-II connector; about 60 ohms means both terminating resistors and the wiring are intact on most high-speed CAN systems.
High-speed CAN networks use two 120 ohm terminating resistors, one at each end of the bus. Because they are in parallel, a healthy network measures about 60 ohms between CAN high and CAN low.
Disconnect the battery or wait for the network to sleep, since live voltage can upset the reading. Measure between pins 6 and 14 on vehicles that use those pins for high-speed CAN, and confirm the pinout in service information. About 120 ohms means one resistor or one branch of the bus is open. A very low reading means the wires are shorted together or a module has failed internally. An open reading means the connector is not wired to the bus, or both sides are open.
Resistance testing is a good start but does not prove the network is healthy. Follow up with a scope to look at the actual signals, since noise, shorts to voltage and bad modules show up there.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.