COM
Laars Communication Bus Fault

Boiler displays COM or Communication Error; may affect cascade or remote control operation.

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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 miswired communication cable between Laars boilers, Incorrect polarity on RS-485 or similar bus, Faulty remote control or cascade module, Electrical noise or grounding issues

How to Fix / Troubleshooting

Safety first: Turn off power to all connected boilers before working on communication wiring.

  • Check wiring topology: For cascaded Laars boilers, verify that the communication cable is daisy-chained as per the installation manual, not star-wired.
  • Verify polarity: Ensure the + and - terminals (e.g., A/B or +/–) are consistent from boiler to boiler. Reversed polarity can cause COM errors.
  • Inspect cable: Look for damage, loose connections, or corrosion at terminals. Tighten all screws.
  • Isolate devices: Disconnect all but one boiler or remote device and power up. Add devices one at a time to identify a faulty unit or segment.

When to call a professional: Complex cascade or building management system integration issues should be handled by a Laars-experienced technician.

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Repair Difficulty

Medium 3/5

Required Part

Communication Harness
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