Boiler locks out with code 101, no heating or hot water, red fault LED on

Safety first: Turn off the boiler at the mains and isolate the gas supply before removing any covers. Allow the boiler to cool fully.

• Check system pressure: Ensure the pressure on the gauge is around 1.0–1.5 bar when cold. If low, top up via the filling loop according to the user manual and bleed radiators.
• Bleed air from the system: Use a radiator key to bleed all radiators starting from the lowest floor. Re-pressurise as needed.
• Check pump operation: With power restored and demand for heat, place a hand on the pump (e.g. Ariston circulation pump assembly). It should vibrate slightly and get warm. If it is hot but not vibrating, the rotor may be seized. Try freeing it by removing the pump head and gently turning the rotor with a screwdriver (power off first).
• Inspect isolation valves and bypass: Confirm that the flow and return isolation valves under the boiler are fully open. Check any automatic bypass valve is not fully closed.
• Check for sludge/blockage: If some radiators are cold and pipes near the boiler are very hot, the primary heat exchanger may be partially blocked with sludge. This usually requires a professional power flush or heat exchanger replacement.
• Reset the boiler: After resolving circulation issues, press the reset button or follow the reset procedure in the Ariston manual. If code 101 returns repeatedly, contact a Gas Safe registered engineer.