BoilerFaultCodesYour boiler is having trouble communicating with its internal pump, which means water cannot circulate properly to heat your home.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Control mode, max. pump speed - Communication error-variable speed internal pump
What causes this fault?
This fault occurs when the boiler's main brain loses its connection with the internal pump that pushes hot water around your radiators. The most common reason is a mechanical failure within the pump itself, though it can also be caused by loose or damaged wiring that prevents the control board from sending signals to the pump.
This fault often appears at the start of autumn when the heating is turned on after a long summer break, as pumps can seize if they haven't moved for several months.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like C5 — no call-out fees, no surprises.
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Viessmann Vitodens 333 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.
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