BoilerFaultCodesYour boiler is currently running in a special engineer-only testing mode for checking gas emissions and is not experiencing a mechanical fault.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: combustion analysis mode (CO mode)
What causes this 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).
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like CO — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Vokera Syntesi E is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.