Boiler displays 104, shuts down shortly after starting, flow temperature rises very quickly

Safety: Isolate power and allow boiler to cool before working on it.

• Check isolation valves: Confirm that the flow and return isolation valves under the boiler are fully open. Partially closed valves can cause rapid temperature rise.
• Bleed air: Bleed the pump (if it has a bleed screw) and radiators to remove airlocks. Refill and repressurise the system.
• Inspect pump: With power on and demand present, check that the Ariston pump is running. If silent and cold, check the electrical connector. If powered but not spinning, the rotor may be seized and the pump head may need replacement.
• Check flow NTC sensor: Locate the flow temperature sensor (NTC) on the flow pipe or heat exchanger. Inspect wiring and connector. If the sensor is loose from the pipe or corroded, it may give false readings. Reseat or replace as needed.
• Reset: After resolving circulation or sensor issues, reset the boiler. If 104 recurs, call a qualified engineer to test sensors and pump current draw.