84 Medium

Keston 84 Fault Code

The Keston 84 fault code means: your boiler has lost the signal from the sensor that monitors your hot water tank temperature, meaning it cannot accurately control your hot water heating.. It appears on 2 Keston models in our database.

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: CYLINDER THERMISTOR FAULT
Typical repair cost £120-220 (including labour)

Engineer required

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

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Keston models that show the 84 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Keston 84 fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026