Thermostat shows E5 and cannot control heating properly

Safety first: Turn off power at the breaker before accessing wiring.

Steps to troubleshoot E5 (external sensor fault):

• 1. Confirm external sensor usage:
  • Check the thermostat configuration to see if an external/remote sensor is enabled.
  • If no external sensor is installed, disable this option in the settings to clear the error.
• 2. Inspect wiring:
  • If an external sensor is installed, remove the thermostat from the wall.
  • Locate the external sensor terminals and verify both wires are firmly secured.
  • Ensure there are no stray strands touching adjacent terminals.
• 3. Measure sensor resistance:
  • Disconnect the external sensor wires.
  • Use a multimeter to measure resistance across the sensor leads.
  • Compare the reading to the sensor's datasheet (often 10k NTC). Infinite or near-zero resistance indicates a fault.
• 4. Check cable path:
  • Inspect any accessible sections of the sensor cable for damage, staples, or crushing.
  • Repair or replace damaged cable runs.
• 5. Replace sensor if needed:
  • Install a compatible Fibaro or specified NTC remote sensor.
  • Reconnect to the thermostat and re-enable external sensor mode in the settings.

When to call a professional: If the sensor cable is routed through inaccessible areas or you are unsure of the wiring path, an electrician or installer should trace and repair the circuit.