Thermostat shows a temperature that is clearly incorrect (e.g. 5°C or 40°C in a normal room).

Safety first: Turn off power before removing the thermostat from the wall.

Steps to check:

• Compare with reference: Use a reliable room thermometer placed near the thermostat to compare readings.
• Check location: Ensure the thermostat is not near a window, external door, or above a radiator or pipe chase. Relocate if necessary.
• Adjust calibration: In the installer menu, adjust the temperature offset (e.g. +/−3°C) to match the reference thermometer, within the allowed range.
• Inspect for drafts: Remove the thermostat and check for cold air coming through the cable hole. Seal the hole with non-hardening putty to prevent drafts affecting the sensor.

Resolution: If large errors remain after relocation and calibration, the internal sensor is likely faulty. Replace the thermostat head.