Safety first: Isolate power to the thermostat before opening the unit or junction boxes.

Steps to resolve:

1. Check for condensation: If the thermostat is mounted on a cold external wall, remove it and inspect for condensation or water droplets on the PCB and sensor area. Allow to dry completely and improve ventilation or insulation behind the thermostat.
2. Inspect external sensor wiring: If using a remote sensor, remove the sensor from the wall and check that the two sensor wires are not touching or crushed. Re‑terminate with proper insulation and ensure no copper is exposed.
3. Test sensor resistance: With power off, disconnect the sensor wires from the thermostat terminals. Measure resistance across the sensor. A reading near 0 Ω indicates a shorted sensor that must be replaced.
4. Replace thermostat head: If the internal sensor is shorted and no external sensor is used, the thermostat head will usually need replacement as the NTC is board‑mounted.

When to seek help: If you cannot access or identify the sensor wiring safely, contact a heating professional.

F4 error and thermostat shows very high or fluctuating room temperature

Possible Causes