AUX means auxiliary heat. On a heat-pump system, the thermostat may call for backup heat when the heat pump needs help reaching the set point. Seeing AUX briefly during a cold morning or after a large thermostat change does not, by itself, prove a fault.
A heat pump normally moves heat into the home. Many installations also have a supplemental heat source. The thermostat decides when to use it based on the setup and conditions. The backup may be electric resistance heat or another configured source; the label on the screen does not tell you which one is installed.
Texas has different winter conditions. A Panhandle cold front is not the same as a mild Gulf Coast morning. If AUX appears while the house is warming and the temperature moves toward the set point, note the duration and weather. Also check whether someone recently raised the set point several degrees. Sudden large changes can call for extra heat on some systems.
AUX and EM HEAT are not interchangeable labels. Emergency heat is usually a separate mode that bypasses normal heat-pump operation in many systems. Do not switch to it simply because AUX appears. Follow the thermostat and equipment instructions; the exact controls vary by model.
If AUX stays on for a long time while the house gets colder, the outdoor unit is not operating as expected, or breakers repeatedly trip, record what you see and arrange a professional assessment. You can check the HEAT setting, the room reading and an accessible filter first, without opening equipment or touching wiring.
If your system is a gas furnace without a heat pump, the screen and heating sequence may differ. Identify the equipment before applying heat-pump advice. A carbon monoxide alarm or suspected gas leak calls for immediate emergency action rather than a thermostat experiment.
To make a service call more useful, write down when AUX appeared, the outdoor temperature, your set point and whether the rooms warmed. Those observations are more precise than saying the thermostat is broken. If you rent, send the same notes to the property contact and follow the building’s maintenance process. Do not reset breakers repeatedly or work inside the unit.