CAN Bus: diagnosing CAN-H, CAN-L and the 60 Ω reading
A “No Communication” DTC does not establish that an ECU is damaged. Before replacing modules, check power supply, grounds and the network's physical layer. CAN lets multiple ECUs share information over a common bus; High-Speed CAN uses CAN-H and CAN-L.
First identify the network you are measuring
Not every network called CAN has the same physical layer. Do not automatically assume the same values or topology for High-Speed CAN, fault-tolerant CAN and CAN FD. Confirm the diagram, bus, participating modules, splices, gateway, speed and terminations before interpreting a measurement.
Classic High-Speed CAN normally uses a CAN-H/CAN-L pair and terminations at the bus ends. CAN also underpins higher-level protocols such as OBD-II, UDS and ISO-TP.
The 120 Ω terminations and why we usually measure about 60 Ω
A conventional High-Speed CAN topology uses a termination resistor of approximately 120 Ω at each end. With the network de-energized, the two terminations are electrically in parallel, so a measurement between CAN-H and CAN-L is usually close to 60 Ω.
Important: 60 Ω does not certify a healthy network. It only indicates that the equivalent resistance at the measurement point is consistent with two terminations being present. Poor ECU power supply, corrosion, high-resistance splices, defective branches, interference, reflections or intermittent faults may still exist.
The correct resistance-measurement procedure
Switch off the vehicle and allow modules to enter sleep according to the manufacturer's procedure. Do not measure resistance on an energized circuit. Verify that voltage is absent before switching the multimeter to ohms, then measure between the CAN lines of the identified bus.
As guidance for a conventional network: about 60 Ω is consistent with two 120 Ω terminations; about 120 Ω may suggest a missing termination or an open circuit separating part of the network; very low values may suggest a short or additional resistance. These readings are clues, rather than a verdict: OEM topology takes precedence.
CAN-H and CAN-L: do not diagnose by DC voltage alone
A multimeter gives an average and can identify obvious shorts to ground or B+, but does not show the bit waveform. High-Speed CAN works differentially: the lines separate electrically to represent the dominant state and return toward their recessive state.
Approximate values commonly observed on High-Speed CAN are CAN-H near 2.5 V in the recessive state, rising toward ~3.5 V when dominant, while CAN-L starts near 2.5 V and drops toward ~1.5 V. These are indicative values, not universal specifications for every CAN architecture.
The oscilloscope reveals what the multimeter hides
To assess communication integrity, observe CAN-H and CAN-L simultaneously. Check activity, amplitude, differential symmetry, edges, noise, distortion, ringing and levels that fail to reach the dominant/recessive states correctly. A network can show apparently correct resistance and still have a poor waveform.
The differential signal is particularly useful: a disturbance common to both lines can largely cancel at the receiver, one reason for CAN's resistance to automotive electrical noise.
When one module can bring down the whole network
A defective transceiver, incorrect power supply, water in a connector or a shorted branch can affect several nodes and generate multiple communication DTCs. Do not replace every module listed as “offline”. Use the topology to identify which modules are absent together and which segment, gateway, supply or splice they share.
Where the OEM procedure permits, isolating branches or nodes can locate the element collapsing the bus. Before disconnecting modules, confirm the safety, wake-up and programming consequences.
Recommended diagnostic order
1) complete scan and map of present/absent modules; 2) battery and stable power supply; 3) bus diagram and topology; 4) power supplies and grounds of critical modules/gateway; 5) check CAN-H/CAN-L shorts to B+ or ground; 6) termination resistance with the network de-energized; 7) oscilloscope waveform; 8) isolate the suspected branch or node following OEM documentation; 9) repair, then repeat the scan and measurement.
Common workshop mistake
Measuring about 60 Ω and concluding “CAN is good”. Termination resistance is only one physical-layer test. It does not establish power supply to every node, dynamic signal quality or correct communication between ECUs.
Conclusion
Professional CAN diagnosis combines topology + power supply + resistance + oscilloscope + module behavior. The objective is to locate and establish the cause of lost communication, rather than guess which ECU is damaged.
Technical reference: developed by Americopp using the CSS Electronics CAN Bus guide as study material and automotive network-diagnosis principles. Electrical values are typical references; the vehicle's OEM diagram and specifications always take precedence.