Resideo
Resideo Smart Thermostats Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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Aux Heat Error
Thermostat shows 'Aux Heat Error' or auxiliary heat does not engage when expected.
Auxiliary Heat Stage Control Failure
Cool On (Flashing)
Display shows 'Cool On' flashing, and the AC does not start.
Compressor Short Cycle Protection / Minimum Off Timer Active
Dehumidifier Error
Thermostat indicates 'Dehumidifier Error' or cannot control the whole-home dehumidifier.
Dehumidifier Control or Communication Fault
E1
Thermostat displays E1 and will not control heating or cooling
Indoor Temperature Sensor Open Circuit
E2
Thermostat displays E2, and the temperature reading is frozen or missing.
Indoor Temperature Sensor Short Circuit
E3
Thermostat displays E3, and the outdoor temperature is not available.
Outdoor Temperature Sensor Open Circuit
E4
Thermostat displays E4, and the outdoor temperature reading is incorrect or missing.
Outdoor Temperature Sensor Short Circuit
E42
Resideo smart thermostat shows E42 and cannot connect to Wi‑Fi.
DHCP Server Unavailable or IP Address Assignment Failure
E43
Thermostat displays E43, and the app cannot connect to the thermostat.
Lost Internet Connection to Thermostat
E99
Thermostat displays E99 and will not operate HVAC equipment.
Internal Thermostat Hardware Failure
Fan Error
Thermostat indicates 'Fan Error' or cannot control the fan independently.
G Terminal Output or Fan Relay Control Failure
Heat On (Flashing)
Display shows 'Heat On' flashing, and the furnace does not start.
Burner Short Cycle Protection / Minimum Off Timer Active
Heat Pump Error
Thermostat indicates 'Heat Pump Error' or auxiliary heat runs constantly.
Heat Pump Control or Reversing Valve Fault
Low Battery
Thermostat displays 'Low Battery' or beeps periodically.
Thermostat Backup Power Depleted
No C Wire Detected
Thermostat reports 'No C wire' or requires constant battery replacement.
Missing Common Wire for Smart Thermostat Power
No Power to R
Thermostat reports 'No Power to R' or loses display power intermittently.
Loss of 24VAC Control Voltage from HVAC Transformer
No Signal
Wireless Resideo smart thermostat or remote sensor shows 'No Signal' or 'Lost Connection'.
Wireless Link Loss Between Thermostat and Equipment Interface or Sensor
No Wi‑Fi
Thermostat shows 'No Wi‑Fi' or a Wi‑Fi icon with an X, and cannot be controlled from the app.
Wi‑Fi Radio Disconnected or Disabled
Schedule Error
Thermostat shows 'Schedule Error' or fails to follow the programmed schedule.
Corrupted Schedule Data or Misconfiguration
Sensor Error
Thermostat or app shows 'Sensor Error' for a specific room or remote sensor.
Remote Room Sensor Communication or Hardware Failure
Symptom
Thermostat backlight stays on constantly or will not turn on.
Backlight Control or Display Setting Fault
Symptom
Thermostat calls for cooling but AC does not start
Cooling Call Control Circuit Failure
Symptom
Thermostat calls for heat but furnace does not start
Heat Call Control Circuit Failure
Symptom
Thermostat cannot be added to the Resideo/Honeywell Home app, or pairing fails repeatedly.
Cloud Pairing or Account Linking Failure
Symptom
Thermostat display is on but unresponsive to touch or buttons
User Interface or Capacitive Touch Failure
Symptom
Thermostat does not control humidity as expected (too humid or too dry)
Humidity Control Misconfiguration or Sensor Error
Symptom
Thermostat does not respond to voice assistants (Alexa, Google, etc.).
Third-Party Integration or Cloud Communication Issue
Symptom
Thermostat frequently disconnects from Wi‑Fi or app shows it offline intermittently
Intermittent Network Connectivity
Symptom
Thermostat is completely blank or dead, no display.
Loss of Thermostat Power
Symptom
Thermostat loses time or schedule after power outages.
Insufficient Backup Power or Configuration
Symptom
Thermostat makes clicking noises frequently, even with small temperature changes
Excessive Relay Cycling
Symptom
Thermostat or wall area feels unusually warm to the touch
Localized Heating from Electronics or Wall Cavity
Symptom
Thermostat overshoots set temperature, room gets too hot or too cold
Improper Cycle Rate or Anticipator Settings
Symptom
Thermostat randomly switches between heat and cool, or runs both in short succession.
Improper Auto-Changeover or System Configuration
Symptom
Thermostat reboots or resets randomly during operation
Intermittent Power or Firmware Instability
Symptom
Thermostat shows temperature that is clearly inaccurate compared to other thermometers
Temperature Sensing Calibration Error
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| Aux Heat Error |
Thermostat shows 'Aux Heat Error' or auxiliary heat does not engage when expected.
|
Auxiliary Heat Stage Control Failure | View Details |
| Cool On (Flashing) |
Display shows 'Cool On' flashing, and the AC does not start.
|
Compressor Short Cycle Protection / Minimum Off Timer Active | View Details |
| Dehumidifier Error |
Thermostat indicates 'Dehumidifier Error' or cannot control the whole-home dehumidifier.
|
Dehumidifier Control or Communication Fault | View Details |
| E1 |
Thermostat displays E1 and will not control heating or cooling
|
Indoor Temperature Sensor Open Circuit | View Details |
| E2 |
Thermostat displays E2, and the temperature reading is frozen or missing.
|
Indoor Temperature Sensor Short Circuit | View Details |
| E3 |
Thermostat displays E3, and the outdoor temperature is not available.
|
Outdoor Temperature Sensor Open Circuit | View Details |
| E4 |
Thermostat displays E4, and the outdoor temperature reading is incorrect or missing.
|
Outdoor Temperature Sensor Short Circuit | View Details |
| E42 |
Resideo smart thermostat shows E42 and cannot connect to Wi‑Fi.
|
DHCP Server Unavailable or IP Address Assignment Failure | View Details |
| E43 |
Thermostat displays E43, and the app cannot connect to the thermostat.
|
Lost Internet Connection to Thermostat | View Details |
| E99 |
Thermostat displays E99 and will not operate HVAC equipment.
|
Internal Thermostat Hardware Failure | View Details |
| Fan Error |
Thermostat indicates 'Fan Error' or cannot control the fan independently.
|
G Terminal Output or Fan Relay Control Failure | View Details |
| Heat On (Flashing) |
Display shows 'Heat On' flashing, and the furnace does not start.
|
Burner Short Cycle Protection / Minimum Off Timer Active | View Details |
| Heat Pump Error |
Thermostat indicates 'Heat Pump Error' or auxiliary heat runs constantly.
|
Heat Pump Control or Reversing Valve Fault | View Details |
| Low Battery |
Thermostat displays 'Low Battery' or beeps periodically.
|
Thermostat Backup Power Depleted | View Details |
| No C Wire Detected |
Thermostat reports 'No C wire' or requires constant battery replacement.
|
Missing Common Wire for Smart Thermostat Power | View Details |
| No Power to R |
Thermostat reports 'No Power to R' or loses display power intermittently.
|
Loss of 24VAC Control Voltage from HVAC Transformer | View Details |
| No Signal |
Wireless Resideo smart thermostat or remote sensor shows 'No Signal' or 'Lost Connection'.
|
Wireless Link Loss Between Thermostat and Equipment Interface or Sensor | View Details |
| No Wi‑Fi |
Thermostat shows 'No Wi‑Fi' or a Wi‑Fi icon with an X, and cannot be controlled from the app.
|
Wi‑Fi Radio Disconnected or Disabled | View Details |
| Schedule Error |
Thermostat shows 'Schedule Error' or fails to follow the programmed schedule.
|
Corrupted Schedule Data or Misconfiguration | View Details |
| Sensor Error |
Thermostat or app shows 'Sensor Error' for a specific room or remote sensor.
|
Remote Room Sensor Communication or Hardware Failure | View Details |
| – |
Thermostat backlight stays on constantly or will not turn on.
|
Backlight Control or Display Setting Fault | View Details |
| – |
Thermostat calls for cooling but AC does not start
|
Cooling Call Control Circuit Failure | View Details |
| – |
Thermostat calls for heat but furnace does not start
|
Heat Call Control Circuit Failure | View Details |
| – |
Thermostat cannot be added to the Resideo/Honeywell Home app, or pairing fails repeatedly.
|
Cloud Pairing or Account Linking Failure | View Details |
| – |
Thermostat display is on but unresponsive to touch or buttons
|
User Interface or Capacitive Touch Failure | View Details |
| – |
Thermostat does not control humidity as expected (too humid or too dry)
|
Humidity Control Misconfiguration or Sensor Error | View Details |
| – |
Thermostat does not respond to voice assistants (Alexa, Google, etc.).
|
Third-Party Integration or Cloud Communication Issue | View Details |
| – |
Thermostat frequently disconnects from Wi‑Fi or app shows it offline intermittently
|
Intermittent Network Connectivity | View Details |
| – |
Thermostat is completely blank or dead, no display.
|
Loss of Thermostat Power | View Details |
| – |
Thermostat loses time or schedule after power outages.
|
Insufficient Backup Power or Configuration | View Details |
| – |
Thermostat makes clicking noises frequently, even with small temperature changes
|
Excessive Relay Cycling | View Details |
| – |
Thermostat or wall area feels unusually warm to the touch
|
Localized Heating from Electronics or Wall Cavity | View Details |
| – |
Thermostat overshoots set temperature, room gets too hot or too cold
|
Improper Cycle Rate or Anticipator Settings | View Details |
| – |
Thermostat randomly switches between heat and cool, or runs both in short succession.
|
Improper Auto-Changeover or System Configuration | View Details |
| – |
Thermostat reboots or resets randomly during operation
|
Intermittent Power or Firmware Instability | View Details |
| – |
Thermostat shows temperature that is clearly inaccurate compared to other thermometers
|
Temperature Sensing Calibration Error | View Details |
No matching codes found
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