Thermostat displays E4 and may shut down heating or cooling

Safety: This code usually indicates an internal thermostat condition, not a fire hazard, but always investigate any signs of overheating in the HVAC system.

• Step 1 – Check location: Ensure the thermostat is not installed above supply registers, near lamps, electronics, or in direct sunlight. If it is, consider relocating the thermostat to an interior wall.
• Step 2 – Shade the thermostat: Temporarily shade the thermostat from sunlight and allow 30–60 minutes to see if the error clears.
• Step 3 – Inspect sub-base: Remove the thermostat from the wall and check for hot air leaking from the wall cavity (e.g., from an unsealed return duct). Seal gaps with foam or caulk.
• Step 4 – Reboot: Power cycle the thermostat by removing it from the sub-base or turning off the HVAC breaker for 2–3 minutes, then restoring power.
• Step 5 – If E4 recurs in normal conditions: The internal temperature sensor or electronics may be failing. Replacement of the thermostat is typically required.