91 High

Keston 91 Fault Code

The Keston 91 fault code means: your boiler has detected a problem with the internal sensor that monitors the temperature of water flowing through the system, preventing it from heating up.. It appears on 1 Keston model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: NO WATER FLOW THERMISTOR FAULT - CONTACT INSTALLER
Typical repair cost £100-£180 (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 91 fault code

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

Source

Keston 91 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