CAN Bus Diagnostics: Common Failure Modes and How to Prevent Them

CAN and CAN FD networks are robust by design, yet a large share of field failures trace back to a small set of repeatable causes: incorrect termination, topology violations, ground loops, marginal power, and connector degradation. This article summarises the most common failure modes seen in industrial and automotive installations and the practical countermeasures that keep networks healthy.

Failure Mode 1 - Missing or Incorrect Termination

Each end of a CAN bus must be terminated with 120 ohm. Missing termination causes reflections and intermittent errors that appear only at high data rates or with long harnesses. Verify termination at both physical ends of the bus, not at intermediate nodes, and measure the DC resistance between CANH and CANL (should read about 60 ohm on a correctly terminated bus).

Failure Mode 2 - Stub Length Violations

Long stubs to individual nodes create impedance discontinuities. At 500 kbit/s keep stubs under roughly 1 m; for CAN FD data rates above 2 Mbps, stubs should be as short as possible or eliminated with point-to-point wiring. Use a star topology with a central hub or repeater when nodes are physically distributed.

Failure Mode 3 - Ground Potential Differences

When nodes are powered from different supplies or located across large installations, ground potentials can differ by volts. This shifts the common-mode voltage of the bus and can saturate the receiver. Galvanically isolated transceivers (for example the TI TCAN1044A-Q1 with an isolated supply or an external digital isolator stage) decouple the bus reference and eliminate this failure class.

Failure Mode 4 - EMC and Electrical Fast Transients

Contactors, motors, inverters and chargers inject fast transients into the harness. Unprotected transceivers can latch up or lose frames. Add TVS diodes on CANH/CANL, use common-mode chokes, and keep the bus wiring away from power conductors. Verify with IEC 61000-4-4 (EFT) and 61000-4-2 (ESD) tests.

Failure Mode 5 - Connector and Harness Wear

Vibration, corrosion and repeated mating degrade contacts over time, raising contact resistance and generating intermittent errors. Use sealed automotive-grade connectors, apply strain relief, and schedule periodic harness inspection in harsh environments.

Diagnostic Approach

• Record error frames and bus-off events with a CAN analyser before changing hardware
• Measure CANH/CANL DC levels, recessive and dominant voltages
• Check termination resistance while the bus is powered down
• Inspect physical layer with an oscilloscope at the worst-case node
• Isolate one segment at a time to locate the offending node

Conclusion

Most CAN failures are preventable with disciplined termination, topology and grounding, plus galvanic isolation where nodes cross supply domains. Applying these practices during commissioning saves significant field service cost over the life of the installation.

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