BoilerFaultCodesYour boiler has detected that the water temperature is changing too quickly or unevenly, usually because the water isn't flowing 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: Variation on temperature of primary and/or return too high
What causes this fault?
This fault occurs when the boiler detects that heat isn't being carried away from the burner quickly enough, causing a sudden, unsafe rise in temperature. The most common reasons are a pump that has seized or failed, a blockage in the internal pipework, or a buildup of limescale and sludge that restricts the flow of water.
This issue often appears when the heating is first turned on in autumn, as pumps can seize up after being inactive throughout the summer months.
DIY fix possible
Check that your boiler pressure is between 1.0 and 1.5 bar on the pressure gaugeEnsure all radiator valves are fully open to allow water to circulateBleed your radiators to remove any trapped air pockets
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 AL74 — no call-out fees, no surprises.
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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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