Thermostat displays E1 and shows indoor temperature as -- or extremely inaccurate

Safety first: Turn off power to the HVAC system at the breaker before removing the thermostat or opening the indoor unit.

• Step 1 – Reboot thermostat: Remove the thermostat from the wall plate for 30 seconds and reinstall. Check if E1 clears temporarily.
• Step 2 – Check ambient conditions: Ensure the thermostat is not exposed to direct sunlight, drafts, or heat sources that could cause extreme readings. This will not cause E1 directly but can help rule out environment issues.
• Step 3 – Inspect sensor connection: Remove the thermostat from the wall. Look for a small thermistor (temperature sensor) on the back of the display board or connected via a small harness. Ensure the connector is fully seated and free of corrosion or damage.
• Step 4 – Test sensor resistance: With power off, disconnect the sensor and measure resistance with a multimeter. Compare to the iComfort service chart (typically around 10kΩ at 77°F/25°C). Infinite or near‑zero resistance indicates a failed sensor.
• Step 5 – Replace sensor or display: If the sensor is removable, replace the indoor temperature sensor assembly. If it is integrated into the display PCB, the entire thermostat display module must be replaced.
• Step 6 – Restore power and verify: Turn power back on and confirm that E1 is cleared and the temperature reading is stable and accurate.