E3
TP-Link Kasa
HVAC Equipment Communication / Control Output Error
Thermostat displays E3 and system will not start heating or cooling
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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 circuit on R or C power feed, Blown low-voltage fuse on furnace control board, Miswired thermostat terminals, Loose connection at thermostat wall plate
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler and outdoor unit at the breakers before inspecting wiring.
- Check thermostat seating: Remove and reseat the thermostat onto its wall plate to ensure all low-voltage pins are making contact.
- Verify 24V power: At the furnace control board, check the low-voltage fuse (often 3A or 5A automotive-style). Replace if blown, but only after checking for shorts in thermostat wiring.
- Inspect R and C wires: At the thermostat backplate, confirm the R (or Rc/Rh) and C wires are firmly secured and not shorting to other terminals.
- Look for shorts: Inspect the thermostat cable between furnace and thermostat for damaged insulation or crushed sections that could short R to another conductor.
- Reset system: Restore power and wait 5 minutes. If E3 persists, the control output circuitry on the thermostat or the furnace control board may be faulty.
- Next step: If wiring and fuse are good, replace the thermostat. If issue remains, have a technician test the furnace control board.
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Repair Difficulty
Hard
4/5
Required Part
Thermostat Low-Voltage Output Circuit / Furnace Control Board Fuse
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