While you wait for answers, we may have found some information which could help you.
Click here for Saunier Duval PCB Board Boiler
saunier Duval is over pressurising when the heating is on. ?
Saunier Duval timeclock not working?
Saunier Duval F35E auto-vents down to code 21 in minutes?
isotwin condens f 35 e service menu codes?
'f19' error report - Saunier Duval 'Thermoclassic' boiler?
How to repair F4 code?