BoilerFaultCodesKeston C33 Fault Code
The Keston C33 fault code means: your boiler has detected an issue with the air supply or exhaust system, meaning it cannot safely remove combustion gases and has shut down to prevent a hazard.. It appears on 2 Keston models in our database.
Technical description: A room sealed appliance designed for connection via ducts to a horizontal or vertical terminal, which admits fresh air to the burner and discharges the products of combustion to the outside through orifices which, in this case, are concentric. The fan is up stream of the combustion chamber.
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 C33 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston C33 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