BoilerFaultCodesVokera CO Fault Code
The Vokera CO fault code means: your boiler is currently running in a special engineer-only testing mode for checking gas emissions and is not experiencing a mechanical fault.. It appears on 8 Vokera models in our database.
Technical description: combustion analysis mode (CO mode)
What causes the Vokera CO fault?
This code appears when the boiler has been manually put into a dedicated testing mode, typically by an engineer turning a selector switch or pressing a specific button sequence. It is designed to override normal thermostat controls so the boiler runs at a fixed rate, allowing for accurate measurement of gas combustion levels. It is not triggered by a component failure, but rather by the system being told to perform a functional test.
DIY fix possible
Turn the selector dial to the 'OFF/RESET' position.Wait approximately 10 seconds.Turn the selector dial back to your desired setting (Heating/Hot Water).
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Gas Safe Register →Vokera models that show the CO fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vokera CO fault code information on this page is sourced and cross-checked against the official Vokera service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026