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

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.

What the manual says

Manufacturer manual

DHW Inlet thermistor Open or Short Circuit

From this model's manual
Verified against the Navien/Navien Lcb700 installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

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

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All 29 fault codes on the LCB700

Seeing a different code? Every code this model can display, in one place.

E001E003E004E012E016E030E031E046E047E109E110E127E205E302E352E353E407E441E445E480E515E517E594E615E740E782E783E787

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