F21
MTU Onsite Energy CAN Communication Loss

Controller displays F21 – CAN bus communication error with 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

Broken or shorted CAN wiring, Incorrect CAN termination, Faulty engine ECU, Loose CAN connectors

How to Fix / Troubleshooting

Safety first: Power down the control system before unplugging CAN connectors.

  • Inspect CAN cabling: Trace the CAN bus between the MTU controller (e.g., MGC/IntelliVision) and the engine ECU. Look for physical damage, tight bends, or abrasion.
  • Check connectors: Reseat all CAN plugs and inspect pins for corrosion or bending.
  • Verify termination: Ensure that the CAN bus has proper termination resistors at both ends as specified by MTU.
  • Measure resistance: With power off, measure resistance between CAN-H and CAN-L. It should typically be around 60 ohms for a properly terminated bus.
  • ECU status: Check ECU power and status LEDs (if present). If the ECU is not powering up, troubleshoot its power supply.
  • Reboot system: After corrections, power up the system, clear the F21 alarm, and verify communication is restored.
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Repair Difficulty

Hard 4/5

Required Part

Engine Control Unit (ECU)
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