AL21
SDMO CAN Bus Communication Fault

Controller indicates AL21 CAN communication error between SDMO controller and engine ECU.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Loose or damaged CAN wiring harness, Incorrect CAN termination resistors, Faulty engine ECU, Faulty SDMO controller CAN interface

How to Fix / Troubleshooting

Safety first: Power down the generator and disconnect battery before working on communication wiring.

  • Inspect CAN wiring: Locate the CAN H and CAN L twisted pair between the SDMO controller and engine ECU. Check for cuts, abrasions, or crushed sections. Repair or replace as needed.
  • Check connectors: Unplug and re-seat CAN connectors at both the controller and ECU. Look for bent pins or corrosion.
  • Verify termination: Ensure that 120 Ω termination resistors are present at each end of the CAN bus. Measure resistance between CAN H and CAN L (should be ~60 Ω total).
  • Test with known-good module: If available, substitute a known-good controller or ECU to isolate the faulty device.
  • Reset AL21: After restoring communication, clear the alarm and verify that engine parameters are correctly displayed.
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Repair Difficulty

Hard 4/5

Required Part

Engine ECU CAN Harness
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