F3 error and floor temperature reading is extremely high or low, causing heating to shut down.

Safety: Isolate power to the thermostat and boiler before handling wiring.

• Step 1 – Inspect sensor terminals:
  • Remove thermostat from wall plate.
  • Check that the two floor sensor wires are not touching each other outside the terminals.
  • Ensure no stray copper strands are bridging to adjacent terminals.
• Step 2 – Measure resistance:
  • Disconnect the sensor wires from the thermostat.
  • Measure resistance with a multimeter.
  • If the reading is very low (e.g. <1 kΩ) or near 0 Ω, the sensor or cable is shorted.
• Step 3 – Check for moisture damage:
  • If the sensor probe is accessible (e.g. in a floor box), inspect for water ingress.
  • Dry the area and correct any leaks before replacing the sensor.
• Step 4 – Replace sensor:
  • Install a new Protherm NTC floor sensor of the correct specification.
  • Route the cable through the existing conduit, avoiding sharp bends.
  • Connect to the thermostat sensor terminals and tighten screws.
• Step 5 – Verify configuration:
  • Confirm the thermostat is set to the correct sensor type and control mode (floor or room+floor).
  • Restore power and check that F3 is cleared and temperature readings are realistic.