Thermostat shows E2 and heating does not start or stops unexpectedly; app may show floor sensor error on underfloor heating models.

Safety first: Turn off power to the heating circuit and thermostat at the breaker. Verify with a non-contact voltage tester before touching any wiring.

Steps to diagnose and fix:

• Confirm sensor mode: In the thermostat settings, verify that the control mode is set correctly (e.g., Floor sensor or Room + Floor for underfloor heating). If the thermostat is set to use a floor sensor that is not installed, E2 can appear. Change to Room sensor mode if no floor sensor is present.
• Check sensor wiring: Remove the thermostat from the wall plate. Locate the floor sensor terminals (often labeled NTC or S1/S2). Ensure both sensor wires are firmly clamped and not broken or pinched.
• Test sensor resistance: With power off, disconnect the sensor wires and measure resistance with a multimeter. A typical 10k NTC sensor should read around 8–15 kΩ at room temperature. An infinite reading indicates an open circuit; near 0 Ω indicates a short.
• Inspect sensor cable route: If resistance is wrong, inspect the cable along its route (junction box, floor probe conduit). Look for crushed, cut, or water-damaged sections. Replace the sensor if damaged.

Resolution: Replace the external NTC floor sensor if readings are out of range or unstable. Clear the error by restoring power and confirming correct sensor type in the thermostat menu.