Boiler displays A04 and will not start; may be associated with low or zero pressure on the gauge.

Safety first: Allow the system to cool before topping up pressure to avoid scalding from hot water.

• Check pressure gauge: If below 0.8 bar (cold), top up via the filling loop to 1.0–1.5 bar following the Ferroli manual.
• Bleed radiators: After topping up, bleed radiators to remove air, then re-check and adjust pressure.
• Inspect for leaks: Look around radiators, valves, and under the boiler for visible water. Any continuous pressure loss indicates a leak that must be repaired.
• If pressure is correct but A04 persists: The pressure sensor or wiring may be faulty; do not bypass safety devices.
• Engineer checks:
  • Test and replace the water pressure sensor if readings are incorrect.
  • Check the expansion vessel charge and recharge or replace if failed.
  • Locate and repair any hidden leaks in the system.