BoilerFaultCodesYour boiler has got too hot and stopped working to protect itself, likely because the water isn't moving through the system properly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Overheat fault
What causes this fault?
This fault occurs when the internal temperature sensors detect that the water inside the boiler has exceeded a safe limit, triggering an emergency shutdown to prevent damage. The most common reason is a failure in the circulation pump or a blockage in the pipework, which stops heat from being carried away from the burner quickly enough. It can also be caused by 'kettling' due to a build-up of limescale or sludge within the main heat exchanger.
This fault is more frequent in autumn when heating systems are first turned on after being dormant all summer, as pump components are more likely to seize during periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1 and 1.5 bar on the gaugePress the 'Reset' button (flame symbol with a cross) to restart the appliance
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 F5 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Vaillant ecoMAX Pro 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.