BoilerFaultCodesYour boiler has stopped working because the water isn't circulating properly between the pipes, causing a sudden and unsafe temperature gap.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Water shortage, temperature difference between flow and return sensors too large
What causes this fault?
This fault occurs when the water inside your boiler heats up too quickly because it isn't moving around the system fast enough. The most common reason is a failing pump that can no longer push the water through the pipes, or a blockage in the heat exchanger that is restricting flow. When the boiler detects such a large temperature difference between the water going out and the water coming back, it shuts down to prevent internal damage.
This issue often comes to light during the first cold spell of autumn, when a pump that has been idle all summer has seized up or struggled to restart.
DIY fix possible
Check that the boiler pressure is between 1.0 and 1.5 bar on the gaugeEnsure all radiator valves and isolation valves under the boiler are fully openTry bleeding your radiators to remove any trapped air pocketsReset the boiler to see if the fault clears
Find a Gas Safe engineer
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Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F.23 — no call-out fees, no surprises.
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