Common problems guide

The most common Saunier Duval Isofast problems

The faults most likely to send a Saunier Duval Isofast into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
38 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
F21 High DIY-safe

Your boiler has switched itself off because the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the filling loop, which is a silver braided hose usually found underneath the boiler.Open the valves at both ends of the hose to allow mains water to enter the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1 and 1.5 bar.Close both valves tightly and remove the filling loop if it is a detachable type.

Est. cost
£0-90
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for F21
02
F22 High DIY-safe

Your boiler has detected that the water pressure inside the system has risen too high, causing it to shut down for safety.

What to try first

Locate the external filling loop, which is usually a silver braided hose with one or two valves located underneath the boiler.Ensure both valves are turned completely off, as a passing valve can cause the pressure to creep up over time.If the pressure remains high, bleed a radiator in your home to drain a small amount of water until the pressure gauge returns to the green zone (usually 1 - 1.5 bar).

Est. cost
£80-150
Parts
Filling loop, Pressure sensor, Pressure relief valve
Full guide for F22
03
21 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

What to try first

Locate the blue filling loop tap found underneath the boiler unit.Slowly turn the blue tap anti-clockwise to let water enter the system.Watch the pressure gauge on the front of the boiler until it reaches 1.5 bar, then turn the tap firmly off.

Parts
Filling loop, Pressure sensor, Expansion vessel
Full guide for 21
04
F1 High Engineer

Your boiler has failed to light the flame, which means it currently cannot produce any heating or hot water.

Est. cost
£120-£250
Parts
Ignition electrode, Gas valve, Spark generator
Full guide for F1
05
F2 High Engineer

Your boiler is struggling to clear exhaust gases safely, so it has shut down to prevent any risk of fumes entering your home.

Est. cost
£150-300
Parts
Air pressure switch, Flue fan, Venturi tube
Full guide for F2
06
F3 High Engineer

Your boiler is struggling to clear exhaust gases properly, so it has safely shut itself down to prevent any risk.

Est. cost
£150-300
Parts
Air pressure switch, Flue fan, Venturi nozzle
Full guide for F3
Parts most often replaced

on the Saunier Duval Isofast

Filling loopPressure relief valvePressure sensorExpansion vesselIgnition electrodeGas valve

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Saunier Duval shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 38-code list for the Saunier Duval Isofast

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →