Thermostat shows E4 and floor temperature reading is extremely high or low and unstable.

Safety first: Switch off power to the thermostat and heating circuit at the breaker. Verify that no voltage is present at the thermostat terminals.

Steps to diagnose and fix:

• 1. Inspect terminal connections: Remove the thermostat and check that the floor sensor wires are not touching each other outside the terminal clamps. Separate and re-terminate if necessary.
• 2. Measure resistance: Disconnect the sensor wires and measure resistance with a multimeter. A reading close to 0 Ω indicates a short. Compare to the expected NTC value at room temperature.
• 3. Check cable for damage: Examine any accessible length of the sensor cable for signs of crushing, staples, or screws penetrating the insulation. If found, cut out the damaged section and rejoin using a suitable junction box and connectors, or replace the sensor if possible.
• 4. Assess sensor probe: If the sensor is in a floor conduit, gently withdraw it and inspect for moisture or physical damage. Replace with a new Danfoss floor sensor if compromised.
• 5. Reinstall and test: After replacing or repairing the sensor, reconnect to the thermostat, restore power, and verify that the E4 error clears and the floor temperature reading is stable.

When to call a professional: If the sensor is embedded in the floor without conduit or the cable route is unknown, professional assistance is recommended to avoid damaging the floor.