when the thermostat calls for heat the boiler fires up but after a short time it switches off and the fault display code reads F3 most of the time but at other times boiler switches off and the fault reads F3.
when using for hot water only there is no problem

corkball, May 2011

F3 fault

-heat sensor damaged
-check the correct positioning & operation of the heating sensor

-no water circulation in the system
-check the pump

-air in the system
-vent the system

to summarise
check your system pressure at least 1.5bar
bleed all rads
listen for pump running

manwithnoname, May 2011