Display shows A03; water heater stops and may show message related to low water level or dry fire protection.

Safety first: Turn off electrical power and allow the heater to cool before opening any bleed valves.

This alarm indicates the unit believes there is no water or insufficient flow, protecting the immersion heater or heat exchanger from dry firing.

• Step 1 – Check system pressure and fill:
  • Verify that the cold water supply valve to the Nibe water heater is fully open.
  • If connected to a pressurized heating circuit, check the system pressure gauge (typically 1.0–1.5 bar when cold). Refill via the filling loop if low.
• Step 2 – Bleed air:
  • Bleed air from the cylinder’s air vent and any high points in the primary circuit.
  • Use a radiator key or screwdriver as appropriate; have a cloth ready for water drips.
• Step 3 – Check flow/level sensor:
  • Locate the flow switch or level sensor used by your Nibe model (refer to the installation manual).
  • Inspect wiring and connectors for damage or loose contacts.
• Step 4 – Circulation pump check:
  • Ensure the circulation pump is not seized. With power off, remove the pump’s front plug and gently turn the shaft with a screwdriver.
• Step 5 – Reset alarm:
  • Restore power, access the alarm menu, and reset A03.
  • Monitor for normal operation and verify that the tank fills and heats without unusual noises (gurgling, hissing).

If the alarm reoccurs and the system is properly filled and bled, the flow or level sensor may need replacement.