Thermostat displays E3 and heating/cooling output is disabled; internal temperature fluctuates wildly or is stuck

Safety first: Turn off power at the breaker before opening the thermostat.

Steps to troubleshoot:

• Remove the thermostat from the wall and visually inspect the sensor area on the PCB (usually near the air vent slots).
• Look for signs of moisture, corrosion, or conductive dust bridging the sensor leads.
• Gently clean the PCB around the sensor with isopropyl alcohol and a soft brush; allow it to dry completely.
• Check for bent sensor leads touching each other or the metal shield; carefully straighten if needed.
• Measure sensor resistance with a multimeter; if it reads near 0Ω, the sensor is shorted and the board or sensor must be replaced.
• If cleaning and inspection do not resolve the issue, replace the thermostat main board or the entire thermostat unit.

Tip: If the thermostat is installed in a very humid location (bathroom, kitchen), consider relocating it or improving ventilation to reduce condensation.