BoilerFaultCodesTechnical description
Diaphragm
What causes this fault?
This fault occurs when the rubber diaphragm inside the diverter valve perishes or tears, preventing the boiler from detecting when you turn on a hot tap. Because the internal mechanism cannot move correctly to divert heat, the boiler fails to fire up even though the water is running. Over time, the rubber naturally loses its elasticity or develops pinholes due to constant stretching and contact with system debris.
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.
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