BoilerFaultCodesYour boiler has switched itself off because it has detected that the internal temperature has become too hot and reached a dangerous level.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Safety temperature limiter by-pass
What causes this fault?
This fault occurs when the water inside the boiler gets too hot too quickly, triggering a safety sensor to shut the system down to prevent damage. The most common reasons are a pump failure that stops water from moving or a blockage in the heat exchanger that traps heat inside the unit.
This issue often surfaces during the first cold spell of autumn when a pump that has been idle all summer fails to restart.
DIY fix possible
Check that all radiator valves are fully open to allow water to flow.Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).Press the reset button (flame symbol with a cross) once to see if the fault clears.
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.
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Vaillant ecoMAX 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.
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