Display shows A05; water heater stops and may indicate return temperature sensor fault.

Safety first: Turn off power before touching any wiring or sensors.

The return temperature sensor monitors the temperature of water returning from the heating circuit or coil.

• Step 1 – Locate sensor:
  • Find the sensor clipped to the return pipe or inserted into a sensor pocket on the return connection of the Nibe cylinder.
• Step 2 – Check mounting:
  • Ensure the sensor is firmly attached to the pipe with thermal paste or in good contact with the pocket.
  • Re-secure any loose clips or insulation around the sensor.
• Step 3 – Inspect wiring:
  • Follow the cable back to the control board, checking for damage or pinched sections.
  • Disconnect and reconnect the plug to clean contacts.
• Step 4 – Test and reset:
  • Optionally measure resistance to confirm sensor health.
  • Restore power, clear the alarm, and verify that the return temperature reading is plausible.

If the alarm returns immediately with correct wiring and a new sensor, the control board may require replacement.