E21
Wyze Emergency Heat / Aux Relay Circuit Fault

Thermostat shows E21 and emergency heat (Aux/E) does not engage on a heat pump system.

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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

E/Aux wire not connected at Wyze base, E/Aux wire loose at air handler or heat pump control board, Incorrect heat pump configuration in Wyze app, Open E/Aux conductor in thermostat cable

How to Fix / Troubleshooting

Safety first: Turn off power to the HVAC system at the breaker.

  • Confirm emergency heat capability: Verify your heat pump system has an emergency or auxiliary heat terminal (E or Aux) on the control board.
  • Inspect E/Aux at thermostat: Remove the Wyze display and ensure the E/Aux wire is fully inserted into the correct terminal.
  • Check E/Aux at equipment: At the air handler or heat pump control board, confirm the same colored wire is attached to the E or Aux terminal.
  • Check Wyze configuration: In the Wyze app, ensure the system is set as a heat pump with auxiliary heat enabled.
  • Continuity test: With power off and E/Aux disconnected at both ends, test continuity of the conductor. Replace the cable if open.
  • Restore power and test emergency heat: Reconnect wiring, restore power, and switch the thermostat to Emergency Heat mode (if available) to verify operation and clear E21.
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Repair Difficulty

Medium 3/5

Required Part

Thermostat Cable (E/Aux Conductor)
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