204 Emergency

Keston 204 Fault Code

The Keston 204 fault code means: your boiler is detecting a flame inside the unit before it has actually commanded the gas to turn on, which suggests a faulty safety sensor or an internal component failing to shut off properly.. It appears on 4 Keston models in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: Flame On Before Gas Valve On
Typical repair cost £150-300 (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 204 fault code

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

Source

Keston 204 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