F75 Can be simply re-pressuring the boiler, press the minus button below the display, this will give you a reading of the system pressure, if below 1 bar, it will need topping up by using the two grey taps underneath the boiler.
It can also be either the pressure sensor faulty, or a faulty pump (the former being more likely).
Reset the boiler by pressing the button (flame with a cross through it) to reset the boiler, if the fault returns, get an RGI to check the actual cause. It'll be as written above.
Carefree, December 2010
