Thermostat displays E3 and underfloor heating or floor protection functions are disabled.

Safety first: Turn off power to the heating circuit and thermostat at the main isolator or breaker. Confirm power is off before loosening any terminals.

Steps to diagnose and fix:

• 1. Check floor sensor terminals: Remove the thermostat from the wall and locate the floor sensor terminals (often marked "S1/S2" or similar). Ensure both wires are firmly clamped and not broken.
• 2. Test sensor resistance at thermostat: With the sensor wires disconnected, measure resistance across them using a multimeter. Compare to Danfoss floor sensor specifications (commonly 10 kΩ or 15 kΩ at 25°C). An infinite reading indicates an open circuit.
• 3. Inspect accessible cable run: If the sensor cable runs in a conduit to the floor, gently pull a small amount to check for damage or breaks near the thermostat box. Do not force if resistance is felt.
• 4. Check junction boxes: If the sensor is extended via a junction box, open it and verify all connections are secure and not corroded.
• 5. Replace floor sensor: If the sensor is confirmed open and installed in a conduit, pull out the old sensor and feed in a new Danfoss-compatible NTC floor sensor. If the sensor is embedded directly in the screed without conduit, you may need to install a new sensor in the floor surface or use room-sensor-only control mode (if supported).
• 6. Clear error: Reconnect the sensor, restore power, and reset the thermostat if required. Confirm that E3 is cleared and floor temperature is displayed correctly.

When to call a professional: If the sensor is embedded without conduit or you are unsure about cable routing, a heating installer should handle sensor replacement.