Medium Warning
53:3 on Viessmann Vitodens 300-W

Your boiler is having trouble communicating with the pump that fills your separate hot water tank, meaning your home's hot water supply may be affected.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Function relay 2: External cylinder loading pump

What causes this fault?

This fault occurs when the boiler's control system loses its electrical connection to the pump responsible for heating your hot water tank. The most common reasons are a loose or damaged connection in the internal wiring harness, a failure of the relay module that sends power to the pump, or the pump's motor itself drawing too much current and causing a communication error.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

53:1View →53:2View →

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