How to Fix Honeywell Thermostat Says Aux Heat on but Not Working

The Honeywell thermostat can display Aux Heat (auxiliary heat) when a heat pump is active. If the unit shows “aux heat on” but the home isn’t heating properly, several common issues could be at play. This guide explains why aux heat may be engaged, what to check on the thermostat and system, and practical steps to restore comfortable temperatures. The advice covers popular Honeywell models and emphasizes safe, effective troubleshooting for homeowners.

Understanding Aux Heat On Signals And Common Causes

Auxiliary heat is a supplemental heat source used when the heat pump alone cannot meet the desired temperature. It typically engages in cold weather or during high demand. When the thermostat indicates Aux Heat On, the heat pump compressor may run briefly, but the extra electric resistance or alternative heating method should warm the home. Causes for a non-working aux heat include incorrect settings, faulty sensor readings, dirty air filters, refrigerant issues, or a malfunctioning strip heater. Identifying the root cause requires a mix of thermostat review and system checks.

Check Thermostat Settings And System Mode

• Verify the thermostat is in the correct mode for heat. A misconfigured setup can keep aux heat active without delivering warmth.
• Review the temperature differential and hold settings. Large setpoint changes can trigger prolonged aux heat operation.
• Look for a temporary hold or vacation mode that might influence heat behavior.
• Ensure the fan setting is appropriate. A stuck or miswired fan can affect heat distribution even if Aux Heat is on.

Inspect The Heat Pump System For Obvious Issues

• Check outdoor unit and indoor coil for debris or ice buildup. Clean as needed and ensure airflow is unrestricted.
• Inspect air filters. A dirty filter reduces airflow and can cause the system to rely more on aux heat.
• Examine the thermostat wiring to verify no loose connections, especially on model-specific terminals labeled Aux, AUX, or W/Aux.
• Confirm that the reversing valve is functioning. A stuck reversing valve can prevent proper cooling/heating cycles and keep aux heat engaged.

Model-Specific Troubleshooting For Popular Honeywell Thermostats

Different Honeywell models handle aux heat signals in distinct ways. Below are practical checks for common units:

• Honeywell VisionPRO: Ensure the heat pump is selected as the primary heat source, not a fossil-fuel backup. Check the auxiliary heat setting in the installer menu and confirm the AUX relay is operational.
• Honeywell Lyric: Verify that Wi-Fi connected settings reflect the heat pump mode. Reset the system if the Aux Heat indicator remains on without delivering warmth.
• Honeywell T9/T10: Confirm that the system type is set to Heat Pump with Aux Heat. Inspect the outdoor sensor readings; a faulty sensor can misrepresent outdoor temperature and trigger aux heat unnecessarily.
• Honeywell 6000 Series: Check the Aux/E/ AUX terminals for proper connections. Replace the air filters and verify the indoor blower is functioning to improve heat transfer.

Diagnosing And Fixing Common Failures

• Dirty or Clogged Air Filters: Replace filters to restore airflow and reduce the need for Aux Heat.
• Refrigerant Leaks Or Low Charge: A refrigerant issue prevents the heat pump from transferring heat efficiently. Signs include ice buildup on the outdoor coil and reduced heating output. Professional service is required.
• Stuck Reversing Valve: If the valve remains in cooling or heating mode, it impedes proper operation. Mechanical repair or replacement by a technician is usually needed.
• Sensor Or Thermistor Failures: Faulty temperature sensors can misread indoor or outdoor temperatures, causing improper Aux Heat engagement. Calibration or replacement may be necessary.
• Electrical Issues Or Faulty Relay: A failing Aux heat relay can prevent heat output even when the thermostat signals for heat. An HVAC technician should test circuits and replace components as needed.

When To Call A Professional

• The system runs continuously without reaching the setpoint after several hours.
• You notice ice buildup on the outdoor unit or unusual buzzing, humming, or smells near the equipment.
• There are refrigerant odors or visible refrigerant lines with frost, indicating a leak or improper charge.
• Electrical or wiring concerns are suspected, including frequent tripped breakers or a burning smell.
• Thermostat diagnostics indicate sensor or control board failures beyond basic resets.

Preventive Maintenance And Best Practices

• Schedule seasonal maintenance for the heat pump and thermostat compatibility check.
• Replace air filters every 1–3 months, depending on usage and household conditions.
• Keep outdoor unit clear of debris, snow, and vegetation; ensure unobstructed airflow.
• Update thermostat firmware if available and ensure compatibility with your heat pump model.
• Document readings from the thermostat’s diagnostic menus to share with a technician if needed.

Key Takeaways For A Honeywell Aux Heat Issue

Aux heat on a Honeywell thermostat is a normal feature for heat pumps in cold weather or high-demand situations. When it shows on but does not heat effectively, homeowners should verify settings, inspect airflow, check for sensor or valve issues, and assess refrigerant health. Start with simple steps like replacing filters and ensuring correct mode, then escalate to professional service for refrigerant, sensor, or electrical problems. With careful checks, most aux heat problems can be resolved quickly and restore comfortable indoor temperatures.