BoilerFaultCodesTechnical description
Flow switch is not connected or terminals are left disconnected; the boiler will not operate.
What causes this fault?
This fault occurs when the boiler's internal safety sensor fails to detect water circulating through the heat exchanger. The most common reasons are a physically stuck flow switch, a failed circulation pump that isn't moving the water, or a loose electrical connection in the wiring harness. Without this movement, the boiler shuts down immediately to prevent the internal components from overheating and sustaining damage.
This issue often surfaces at the start of autumn when heating systems are turned back on after sitting idle all summer, which can cause the pump or switch to seize.
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.