Display shows A02 and water heater stops heating; unit indicates overheat in tank sensor.

Safety first: Isolate power at the main switch or breaker before removing covers or touching wiring.

This alarm is related to the tank temperature sensor (often an NTC sensor in a dry well).

• Step 1 – Visual inspection:
  • Remove the front cover to access the sensor pocket on the cylinder.
  • Check that the sensor is fully inserted into the sensor well and secured with its clip.
  • Inspect the sensor cable for cuts, crushed insulation, or loose connectors at the control board.
• Step 2 – Reseat connectors:
  • Disconnect the sensor plug from the control board, inspect for corrosion, then reconnect firmly.
• Step 3 – Basic sensor test (if you have a multimeter):
  • With power still OFF, disconnect the sensor.
  • Measure resistance across the sensor leads; compare to Nibe’s NTC resistance table (e.g. around 10 kΩ at 25 °C – check your manual).
  • If the reading is open circuit, shorted, or far from expected, the sensor is likely faulty.
• Step 4 – Reset alarm:
  • Restore power and clear the alarm via the user interface (menu > alarms > reset).
  • Monitor the displayed tank temperature; if it jumps erratically, replace the sensor.

If the alarm persists with a known-good sensor, the control board may be defective and should be checked by an authorized Nibe technician.