BoilerFaultCodesTechnical description
Primary flow switch not operated. Check CH system pressure, automatic air vent, or primary flow valve diaphragm.
What causes this fault?
This fault occurs when the boiler's internal sensors detect that water isn't circulating through the heat exchanger, causing a safety shutdown to prevent internal damage. The most common reason is a lack of water pressure in the system, but it can also be caused by a mechanical failure in the pump or a perished rubber diaphragm that fails to trigger the flow switch.
This issue is more frequent in autumn when the heating is turned on for the first time after summer, as pump components can seize during long periods of inactivity.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.