BoilerFaultCodesYour boiler is overheating because the water is not flowing through the system quickly enough to be cooled down.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow/Return Differential greater than 50°C
What causes this fault?
This fault occurs when the water temperature leaving the boiler is significantly higher than the water returning, usually because something is blocking the flow. The most common reasons are a failing pump that cannot push the water around your radiators or a buildup of sludge and limescale inside the pipework. When the water sits still or moves too slowly, the heat builds up too fast for the boiler to handle safely.
This issue is most common during the first cold spell of autumn, as pump rotors often seize up after being left idle throughout the summer months.
DIY fix possible
Ensure all radiator valves (TRVs) are turned to the highest settingCheck that the boiler pressure is between 1.0 and 1.5 bar on the gaugeSwitch the boiler off and back on at the fused spur to reset the system
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 0U — no call-out fees, no surprises.
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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.
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