First checks
Confirm gas is available — if you can, light a cooker or fire. The handbook lists this as an ignition fault, but the time of year matters. In hard frost it is often a blocked or frozen condensate.
Frozen condensate
Switch the boiler off. Pour warm water (about 50 °C) over the exposed condensate pipe until it runs clear. Switch on and reset if needed. Repeat if the lockout returns.
A temporary check: cut the internal condensate and drop a flexible hose into a bucket (use towels). If the boiler then lights, the outdoor run is still frozen or blocked.
Normal weather
Have the ignition electrodes checked. The gas valve or PCB can also fail on this code and may need testing or replacement.
Gas work stays with a Gas Safe registered engineer. These steps are guidance from the manufacturer sequence, not a substitute for the handbook.