Honeywell TH4210U2002 PRO 4000 5-2 Day Programmable Thermostat AUX-HEAT Error Fix: "Aux Heat" Displayed

Error code AUX-HEAT on the Honeywell TH4210U2002 PRO 4000 5-2 Day Programmable Thermostat means: "Aux Heat" Displayed. The thermostat has activated the auxiliary heat strips because the heat pump alone cannot keep up with heating demand — this is normal in very cold weather but increases energy costs significantly; if Aux Heat runs constantly even in moderate weather, the heat pump may have low refrigerant, a failed compressor, or a frozen outdoor unit

The most successful fix is "Inspect and Service Heat Pump Components to Fix Constant Aux Heat" (hard difficulty, ~90 minutes). Estimated DIY cost: $15-$30. 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 and Service Heat Pump Components to Fix Constant Aux Heat") is rated hard difficulty and takes about 90 minutes. Tools needed: Refrigerant gauge set, Multimeter, Voltage tester, Protective gloves.

DIY cost: $15-$30. Professional repair typically costs $250-$400. Many homeowners save money by trying the top-ranked community fix first.

The Honeywell TH4210U2002 PRO 4000 5-2 Day Programmable Thermostat has 8 other known error codes, including: APP-OFFLINE (Thermostat Shows Offline in App), BLANK-SCREEN (Blank/Dead Display — No Power), BLOWN-FUSE (HVAC Control Board Fuse Blown), COOL-ON-FLASH ("Cool On" Flashing on Display), EM-HEAT ("Em Heat" or Emergency Heat Mode), and 3 more.

Error code AUX-HEAT also appears on 7 other Honeywell models: RTH7600D 7-Day Programmable Thermostat with Backlit Display, RTH9585WF Wi-Fi Color Touchscreen 7-Day Programmable Thermostat, T10 Pro Smart Thermostat with RedLINK Room Sensor, T6 Pro Programmable Thermostat (TH6320WF / TH6220U), T9 Smart Thermostat with Smart Room Sensor, TH6220D1028 FocusPRO 6000 2H/2C Programmable Thermostat, and 1 more. The fix may vary by model, so check each model's specific page for tailored repair steps.