While you wait for answers, we may have found some information which could help you.
Click here for Pressure Losing Replaced
How to repair F4 code?
Saunier D’uval therma classuc?
Saunier Duval Isofast condens F35E....nothing working?
I have a Saunier Duval Isotwin condens F 35 E with hot water No heating?
I need to find the reset switch ?
Saunier Duval F35E auto-vents down to code 21 in minutes?