Observation:

Does the display show a 'Status code' (S) or a 'Fault code' (F)?
Note:
Fault Mode.
Should a fault develop in the boiler, the display will show a 'Fault code' indicating what the problem may be. The fault code appears as a letter 'F' followed by a two digit number e.g. 'F.21'.
Status Mode.
The display may also be used to show the current status of the boiler in its operating cycle. To select status mode press the right hand button once, pressing it a second time will return to normal mode.
The status mode is helpful in diagnosing faults where a fault code is not displayed.