BoilerFaultCodesYour boiler is struggling to move water through the system properly, which is 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: System response fault. Check pump operation and speed setting. Check 3-way valve operation. Purge air from H/E. Check by-pass and other water valves open. Check boiler control responds to flow & ret. sensors
What causes this fault?
This fault occurs when the boiler detects that water isn't circulating quickly enough to carry heat away from the burner, leading to a risk of internal damage. The most common reasons are a pump that has seized or slowed down, a blockage in the central heating pipes, or a diverter valve that is stuck and preventing water flow. Essentially, the boiler is 'choking' on its own heat and shuts down as a safety precaution.
This issue often surfaces in autumn when the heating is turned back on after a long summer break, as internal moving parts like the pump or valves can seize up from lack of use.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like A40 — 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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