Thermostat displays E03 and shows no temperature reading from the room sensor.

Safety first: Isolate power to the thermostat and boiler before removing the thermostat from its base or opening any covers.

Steps to check:

• 1. Reboot thermostat: Remove the thermostat from its wall base (if battery powered) or power it down at the fused spur for 1–2 minutes, then restore power.
• 2. Check location and environment: Ensure the thermostat is not exposed to condensation, direct water splashes, or extreme heat sources that may damage the sensor.
• 3. Inspect for visible damage: With power off, remove the thermostat from its backplate and look for signs of corrosion, moisture, or burnt components near the sensor area.
• 4. Compare readings (if intermittent): If the error is intermittent, compare the displayed temperature with a separate thermometer. Large discrepancies may indicate a drifting NTC sensor.

When to call a professional: The internal NTC sensor is usually integrated on the PCB and not user-serviceable. If E03 persists after a reset and relocation, the thermostat head unit typically needs replacement.