BoilerFaultCodesYour boiler has switched itself off because it has become too hot internally and needs to cool down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Safety switch-off – overheating temperature reached
What causes this fault?
This fault occurs when the water inside the boiler reaches an illegally high temperature, triggering a safety sensor to shut the system down. The most common reason is that the water isn't moving through the unit quickly enough, often due to a pump failure or a blockage caused by sludge buildup in the pipework. It can also happen if there is trapped air in the system or if the internal divertor valve has become stuck.
This fault is more frequent when the heating is first turned on after summer, as pump components can seize up during months of inactivity.
DIY fix possible
Check that your radiator valves are open and not turned to the off positionEnsure your boiler pressure is between 1.0 and 1.5 bar on the displayPress the reset button (a flame symbol with a cross through it) once the boiler has cooled down
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F.20 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.