Flexicom 18 SX boiler - False F9 code

[SaabmanBOF]

Not that long since I replaced the 3-port valve, and now there's another problem...

The boiler doesn't like early morning starts, and refuses to play ball. It displays the F9 code, but the pressure is fine (1.1 bar). Pressing the + and reset buttons simultaneously does not get it to fire up.

If I turn it off at the mains, count slowly to ten, then try again, it fires up. It then runs normally for the rest of the day (three timed periods, not continuously). Next morning, I have to do the same thing again.

Does anybody have any ideas on what may be the cause?

 

[SaabmanBOF]

Changed the pressure switch, and all is well. I can't see how the same signal from the switch could be interpreted as both 1.1 bar and less than 0.5 bar unless there was something else wrong, but there were no codes in the State diagnostic check, so fingers crossed the problem won't recur.