E03 displayed and thermostat cannot detect the external floor sensor or remote probe; underfloor heating may not regulate correctly.

Safety first: Turn off power to the thermostat circuit at the breaker before touching any wiring.

• Verify sensor wiring: Remove the thermostat from its base. Locate the external sensor terminals (often marked S1/S2 or similar). Ensure the two sensor wires are firmly clamped and not broken.
• Check continuity: If you have a multimeter, disconnect the sensor wires from the thermostat and measure resistance across the sensor. A typical Delta Dore NTC floor sensor will show a resistance in the kilo-ohm range (e.g. 10kΩ at 25°C). An infinite reading indicates a broken cable.
• Inspect junction boxes and conduit: If accessible, check any junction box where the sensor cable is spliced. Look for loose connectors or damaged insulation.
• Confirm configuration: In the thermostat menu, verify that an external sensor is enabled: Menu > Installation > Sensor type. If no floor sensor is installed, disable the external sensor option to clear E03.

If the cable is damaged in the floor and cannot be repaired, the only practical solution is to disable the floor sensor mode and use air temperature control, or have a professional install a new sensor in a spare conduit if available.