Johnson Controls
Aux/E Heat Control Failure
Thermostat does not energize auxiliary or emergency heat on a heat pump system when outdoor temperature is very low.
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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
Aux/E terminals not wired, Heat pump system type misconfigured, Failed aux heat relay on thermostat, Outdoor temperature lockout set too high
How to Fix / Troubleshooting
Safety: Turn off power to the air handler/heat pump before working on wiring.
- Step 1 – Verify system type: In the installer menu, confirm the thermostat is configured for a heat pump with auxiliary heat.
- Step 2 – Check wiring: Ensure the auxiliary heat call wire is connected to W2/Aux or E at both thermostat and air handler.
- Step 3 – Review lockout settings: Check any outdoor temperature lockout settings that may prevent aux heat from running above a certain temperature. Adjust as needed.
- Step 4 – Test aux call: Force an auxiliary heat call from the thermostat (emergency heat mode) and verify 24 VAC is present on the aux terminal at the air handler.
- Step 5 – Replace thermostat or control board: If no aux call is sent, replace the thermostat main control board. If aux call is present but heat does not run, have a technician diagnose the air handler heat strips or control board.
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Repair Difficulty
Hard
4/5
Required Part
Aux Heat Relay / Main Control Board
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