BoilerFaultCodesYour boiler has sensed that water is not circulating through the system quickly enough, causing it to shut down to prevent overheating.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: High Flow/Return Differential
What causes this fault?
This fault occurs when the water isn't moving through the boiler fast enough to carry heat away, making the internal temperature rise too quickly. The most common reasons are a failing pump that can't push the water around, or a buildup of sludge and debris that has physically blocked the pipes or the heat exchanger.
This issue often surfaces in autumn when the heating is turned back on for the first time, as pump components can seize or sludge can settle during the summer months.
DIY fix possible
Check that all radiator valves are fully openEnsure there are no blockages or trapped air by bleeding your radiatorsVerify that your system pressure is between 1.0 and 1.5 bar on the pressure gauge
Find a Gas Safe engineer
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A boiler cover plan would cover faults like PP — 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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