Flue gas temperature too high.if you have changed the fan , checked the flue for blockages , make sure the burner is clean to ensure full air flow, then probably worth replacing the flue temp stat. To clear the fail code , turn power to the boiler off for 10 mins, then back on
Have you tried the rest button?
Below the power button
Ok ,in he manual it does say some fault codes won't go till the error is fixed . I would definitely change the flue temp stat first , as it will be relatively cheap.