Your boiler has shut down because the mixture of gas and air required for a safe flame is incorrect, often due to a blockage or a dirty internal component.

Improper gas/air ratio. Check the air restriction and the gas injector for the correct diameter, mounting, and pollution.

What causes this fault?

This fault happens because the boiler cannot achieve the precise balance of gas and oxygen needed for safe combustion. It is typically caused by a buildup of dust or debris in the air intake or a blockage in the internal nozzle that delivers gas to the burner. When the sensors detect that the mixture is unstable or inefficient, the boiler shuts down as a safety precaution.