BoilerFaultCodesThe 6 most common faults
01
21 High DIY-safe
Your boiler has switched itself off because the system water pressure has dropped too low to operate safely.
What to try first
Locate the small blue filling tap underneath the boiler.Slowly turn the tap to allow water into the system while watching the pressure gauge.Close the tap firmly once the needle reaches 1.5 bar.
02
23 High Engineer
Your boiler has detected that water is not circulating through the system correctly, which has caused it to stop working for safety.
Full guide for 2303
04 High Engineer
Your boiler is failing to light the internal flame, which means it cannot produce any hot water or heating.
Full guide for 0404
05 High Engineer
Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.
Full guide for 0505
06 High Engineer
Your boiler has lost the ability to accurately measure the temperature of the water being sent to your radiators, so it has safely shut down to prevent overheating.
Full guide for 0606
09 High Engineer
Your boiler is currently unable to accurately read the water pressure in the system, preventing it from operating safely.
Full guide for 09on the Saunier Duval Isofast F28E
Filling loop tapExpansion vesselPressure sensorCirculation PumpFlow Rate SensorMain PCB
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
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