Diagnosing CAN Faults
From no-communication to intermittent faults: a systematic approach.
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What you will learn
- Localise an open or short on the CAN bus
- Use measured resistance and scope to find the fault
- Decide between wiring, termination and module faults
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- Measure bus resistance at the DLC with ignition off.
- Compare with the expected value for this network.
- If wrong, disconnect one module at a time and re-measure to localise the change.
- Scope CAN-H and CAN-L to confirm activity and check for distortion.
- Wiggle-test harnesses and connectors while watching the scope.
CASE 007
Multiple Modules Offline (CAN Failure)
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The instrument cluster shows several warnings. The engine runs but the ABS, stability and airbag lights are all on. The scanner cannot reach several modules.
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Key points
- Start at the DLC, then isolate branches
- Disconnecting modules changes bus resistance predictably
- Intermittent faults need load and movement tests
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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