Debugging CAN Bus: The 5 Most Common Field Issues

When a CAN bus refuses to work, software is rarely the culprit. In the field, the overwhelming majority of issues fall into these five categories.

1. Termination

You need 120 Ω at both ends of the bus. Measurement: with power off you should read ~60 Ω between CAN_H and CAN_L.

2. Wrong bit timing

The sample point should sit between 75% and 87.5%. Different sample points across nodes cause random error frames on long buses.

3. Missing common ground

Without a ground reference between separately powered nodes, the common-mode voltage exceeds the transceiver limits.

4. A node gone error-passive

Read the TEC/REC counters; a node above 128 silently becomes error-passive and disturbs the bus.

5. Stub length

Long stubs off the main trunk cause reflections; keep them under 30 cm at 1 Mbit/s.