Your boiler is displaying a status message rather than a breakdown code, indicating it is checking water temperatures and simply requires a reset to return to the pressure display.

Indicative flow temperature to DHW. O.K. Scroll to pressure display "P". Press RESET button to scroll display

What causes this fault?

This message appears when the boiler's internal computer is displaying the current water temperature being sent to your taps rather than the system pressure. It is not actually a fault, but rather a specific monitoring mode that occurred because a button was likely pressed or the system is performing a routine check. You simply need to toggle the display settings to return the screen to its standard pressure reading.