BoilerFaultCodesYour boiler has switched itself off because it has become too hot internally, usually due to a lack of water flow or a blockage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Over heat cut off activated | Maximun temperature exeeded
What causes this fault?
This fault occurs when the water inside the boiler reaches a temperature that is dangerously high, triggering a safety shut-off. This is usually caused by a pump failure, a blockage in the pipework, or a build-up of sludge that prevents heat from moving away from the internal heat exchanger. Effectively, the heat is being generated but has nowhere to go, causing the system to protect itself by locking out.
This fault is frequently seen at the start of autumn when heating systems are turned on for the first time in months, as pump components can seize up after being idle all summer.
DIY fix possible
Check that your boiler pressure is between 1.0 and 1.5 barEnsure all radiator valves and isolation valves are fully openPress the reset button (usually 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.
Vaillant Thermocompact is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.