While you wait for answers, we may have found some information which could help you.
Click here for Saunier Hot Water HeatingSaunier Duval Isofast condens F35E....nothing working?
saunier Duval is over pressurising when the heating is on. ?
saunier duval boiler problem?
saunier duval sd620f?
How to repair F4 code?
Saunier Duval F35E auto-vents down to code 21 in minutes?