BoilerFaultCodesKeston E99 Fault Code
The Keston E99 fault code means: your boiler has detected an electrical fault with the sensor that monitors the temperature of the exhaust gases, causing the system to shut down for safety.. It appears on 9 Keston models in our database.
Technical description: Flue sensor short circuit – check connections to flue sensor are in place
What causes the Keston E99 fault?
This fault occurs when the boiler's internal computer detects an electrical failure or a 'short circuit' in the sensor that monitors exhaust temperatures. The most common reason is that the sensor has failed internally due to heat exposure, or the wiring connecting it to the main control board has become damaged or loose.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Keston models that show the E99 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston E99 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 27 April 2026