Display shows A04; water heater does not heat and indicates sensor fault for hot water.

Safety first: Disconnect power at the breaker before accessing the control compartment.

This alarm is associated with the domestic hot water (DHW) temperature sensor.

• Step 1 – Locate DHW sensor:
  • On Nibe cylinders, the DHW sensor is usually inserted into a dedicated sensor well on the tank.
  • Identify the sensor by tracing the cable from the control board to the tank.
• Step 2 – Inspect and reseat:
  • Ensure the sensor is fully inserted into the well and secured.
  • Unplug and replug the connector at the control board to ensure a solid connection.
• Step 3 – Resistance check (optional):
  • With power off, measure the sensor resistance and compare to Nibe’s NTC chart.
  • Replace the sensor if readings are out of range or intermittent when the cable is flexed.
• Step 4 – Clear alarm:
  • Restore power and reset the alarm via the user interface.
  • Verify that the displayed DHW temperature is realistic and stable.

Persistent A04 alarms with a new sensor indicate a possible control board fault that should be diagnosed by a qualified technician.