Ecobee ecobee4 Smart Thermostat AUX-OUTDOOR Error Fix: Auxiliary Heat Outdoor Temperature Alert

Error code AUX-OUTDOOR on the Ecobee ecobee4 Smart Thermostat means: Auxiliary Heat Outdoor Temperature Alert. Auxiliary heat activated when the outdoor temperature exceeded the configured threshold — the heat pump should be able to heat the home without auxiliary heat at this temperature; check heat pump operation, refrigerant charge, and thermostat compressor minimum on/off time settings

The most successful fix is "Inspect Heat Pump Operation and Test Refrigerant Charge" (hard difficulty, ~90 minutes). This fix costs nothing. HomeMD has 2 community-ranked fixes for this error.

This error is rated medium severity. It should be addressed soon, but your appliance may still function in a limited capacity.

Yes — the top-ranked community fix ("Inspect Heat Pump Operation and Test Refrigerant Charge") is rated hard difficulty and takes about 90 minutes. Tools needed: Multimeter, HVAC manifold gauge set, Fin comb, Protective gloves.

DIY cost: $0. Professional repair typically costs $200-$350. Many homeowners save money by trying the top-ranked community fix first.

The Ecobee ecobee4 Smart Thermostat has 8 other known error codes, including: APP-SYNC (App and Thermostat Out of Sync), AUX-RUNTIME (Auxiliary Heat Runtime Alert), BLOWN-FUSE (HVAC Control Board Fuse Blown), COOL-FAIL (Cool Not Reaching Set Temperature), FAN-CONSTANT (Fan Running Constantly), and 3 more.

Error code AUX-OUTDOOR also appears on 5 other Ecobee models: ecobee Smart Thermostat Enhanced with Radar Occupancy Sensor and Large Touchscreen, ecobee Smart Thermostat Essential with Touchscreen Display and eco+ Smart Features, ecobee Smart Thermostat Premium with Siri and Alexa Built-In, Air Quality Monitor, and SmartSensor Included, ecobee SmartThermostat with Voice Control and Alexa Built-In, ecobee3 Lite Smart Thermostat. The fix may vary by model, so check each model's specific page for tailored repair steps.