E2 error on display, thermostat cannot control HVAC, may show incorrect room temperature

Safety: Turn off power to the HVAC system at the breaker before touching any thermostat wiring.

• Identify external sensor: If your Kasa thermostat model supports an external temperature sensor, locate the sensor wiring at the thermostat backplate (often labeled S1/S2 or similar).
• Check connections: Gently tug each sensor wire at the terminal. Tighten any loose screws. Ensure no bare copper is touching adjacent terminals.
• Inspect cable: Follow the sensor cable along the wall or to the remote location. Look for staples through the cable, cuts, or crushed sections. Repair or replace damaged cable.
• Sensor test: If you have a multimeter and know the sensor type (e.g., 10k NTC thermistor), disconnect the sensor and measure resistance. Compare to manufacturer’s temperature/resistance chart. An open (infinite) or short (near 0 Ω) reading indicates a bad sensor.
• Remove sensor (test): Temporarily disconnect the external sensor wires and reboot the thermostat. If E2 clears and the thermostat works using its internal sensor, the external sensor or wiring is faulty.
• Replace if needed: Install a compatible TP-Link Kasa external temperature sensor or equivalent specified by the thermostat documentation.