Thermostat displays E3 and may show incorrect indoor temperature or fail to control system properly.

Safety first: Turn off power to the HVAC system at the breaker to avoid accidental shorting of low-voltage circuits.

• Step 1 – Reboot thermostat: Remove the thermostat from its wall plate for 30 seconds, then reattach. If it has a reset option in the menu, perform a soft reset and recheck for E3.
• Step 2 – Check thermostat location: Ensure the thermostat is not in direct sunlight, above a TV or electronics, or near a supply register. Excess heat can cause sensor errors. If necessary, relocate the thermostat to an interior wall away from heat sources.
• Step 3 – Inspect thermostat contacts: Examine the metal contacts between the thermostat and wall plate. Clean gently with a dry cloth to remove dust or oxidation that could affect sensor readings.
• Step 4 – Compare temperatures: Use a reliable room thermometer placed next to the thermostat. If the thermostat reading is off by more than a few degrees and E3 persists, the internal sensor is likely faulty.
• Step 5 – Replace thermostat: If the internal sensor has failed, the entire Carrier smart thermostat typically must be replaced, as the sensor is integrated into the main board.
• When to call a pro: If you are unsure whether the fault is in the thermostat or the HVAC system, have a Carrier technician verify before replacing the thermostat.