Aux Heat Error
Resideo Auxiliary Heat Stage Control Failure

Thermostat shows 'Aux Heat Error' or auxiliary heat does not engage when expected

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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

Loose Aux/E wiring, Misconfigured auxiliary heat settings, Failed relay on thermostat or air handler board, Overload or open safety limit on electric heat strips

How to Fix / Troubleshooting

Safety first: Turn off power to the air handler/heat pump at the breaker. Electric heat strips can draw high current and are hazardous.

Steps to troubleshoot:

  • 1. Verify wiring: Confirm the Aux/E wire is connected to the correct terminal at both the thermostat and the air handler control board.
  • 2. Check installer setup: In the thermostat's advanced settings, ensure auxiliary heat stages and lockout temperatures are configured correctly for your system.
  • 3. Test in emergency heat: Switch the thermostat to Emergency Heat mode. If no heat is produced, the auxiliary heat circuit or strips may be faulty.
  • 4. Inspect air handler (pro only): A technician should check electric heat strip contactors, fuses, and high-limit switches for open circuits.

When to call a professional: Auxiliary heat issues often involve high-current components; have an HVAC technician diagnose and repair these circuits.

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Repair Difficulty

Hard 4/5

Required Part

Auxiliary Heat Relay or Electric Heat Strips
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