Error 401, furnace will not start, display indicates low system pressure.

Safety: Do not over-pressurize the system. Typical cold pressure is around 1.0–1.5 bar; check your Ariston manual.

• Read the pressure gauge: If below the minimum threshold (often 0.5–0.8 bar), the furnace will lock out with 401.
• Top up the system: Locate the filling loop (flexible hose with two valves) under or near the furnace. Open both valves slowly until the gauge reaches the recommended pressure, then close both valves tightly.
• Bleed air: Bleed radiators starting from the highest floor. Recheck pressure and top up again if needed.
• Check for visible leaks: Inspect radiator valves, pipe joints, and the furnace underside for drips. Persistent pressure loss indicates a leak that must be repaired.
• Pressure sensor check: If the gauge shows normal pressure but 401 persists, the electronic pressure sensor may be faulty and should be replaced.