H4
Day & Night
Communication Loss Between Thermostat and Indoor Unit (Communicating Systems)
H4 code, unit does not respond to thermostat, both indoor and outdoor may be idle
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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
Open or shorted communication bus wiring, Incorrect polarity on communication terminals, Failed communicating thermostat, Faulty indoor control board communication port
How to Fix / Troubleshooting
Safety first: Turn OFF power to the indoor unit and thermostat before working on low-voltage wiring.
- Step 1 – Check thermostat display: If blank or error is shown, verify thermostat has power and is correctly wired.
- Step 2 – Inspect communication wiring: At the indoor board and thermostat, ensure the communication wires (often labeled ABCD or similar) are landed on matching terminals with correct polarity.
- Step 3 – Look for shorts: Examine the cable for damage, staples through the jacket, or moisture in junction boxes.
- Step 4 – Power reset: After correcting wiring, power cycle the system and allow several minutes for devices to re-establish communication.
- Professional help: If H4 persists, either the thermostat or indoor board may need replacement and re-commissioning.
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Repair Difficulty
Medium
3/5
Required Part
Communicating Thermostat
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