Medium
E421 on Navien LCB700

Your boiler is having trouble sensing the temperature of the cold water coming into the system, which means it cannot reliably heat your hot water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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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

DHW Inlet thermistor Open or Short Circuit

Engineer required

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

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