By Buckaroo Heating & Air · November 6, 2026
An unfamiliar AUX message can be unsettling on the first cold morning. It generally indicates that a heat pump’s auxiliary heating is operating. That message by itself does not establish that something is broken.
Auxiliary heat is part of some system designs
Depending on the equipment and controls, supplemental heat may operate when the heat pump needs help meeting demand or during defrost. The exact behavior depends on the installed system. Your thermostat manual and equipment documentation are more useful than a universal outdoor-temperature rule.
Look at what happens around the message
Record the outdoor conditions, room temperature, setpoint, and how long the message remains visible. Note whether someone recently changed the thermostat setting. Also note whether the home reaches its usual comfortable temperature.
A brief display during a cold-weather operating cycle presents a different question from a display that stays on while the house continues to get colder. Give the technician that distinction rather than trying to diagnose a failed component from the screen.
AUX and emergency heat are not interchangeable instructions
Some thermostats offer a separate emergency-heat setting. Do not select it simply because AUX appeared. Ask your technician to explain the available modes for your system and when each should be used. A label on another brand’s thermostat may behave differently.
What to ask during a service visit
- What is the backup heat source in this house?
- When is this system designed to use it?
- Are the controls configured for the actual equipment?
- What operation would justify a follow-up call?
- Is there a written thermostat guide we can keep?
If operation changes suddenly, the system cannot maintain comfort, or the display persists in conditions that were previously normal, arrange professional evaluation. Avoid repeatedly changing modes to force a different outcome.
Our pre-visit checklist can help you prepare the information a technician needs. For an existing heating problem, read about a diagnostic visit.
Reference: Trane explanation of auxiliary heat. This reference explains general operation, not a recommendation to change your equipment settings.