BoilerFaultCodesYour boiler has detected trapped air inside the system which is preventing the water from circulating properly and causing it to overheat and shut down.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Displayed if air passing through the pump leads to boiler shut down (monitored temperature rise across the heat exchanger).
What causes this fault?
This fault occurs when air pockets become trapped within the boiler's heat exchanger, which prevents the water from moving through the system effectively. The most common reason is a failure of the internal air release valve or a buildup of sludge and debris that has hindered the pump's ability to push water around your radiators.
This issue often surfaces in autumn when the heating is turned back on after a long summer break, as air and sediment tend to settle in the pipework while the system is inactive.
DIY fix possible
Check the system pressure gauge and top up to 1.5 bar if lowBleed all radiators in the house to release trapped airRestart the boiler using the reset button
Find a Gas Safe engineer
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A boiler cover plan would cover faults like A — no call-out fees, no surprises.
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