BoilerFaultCodesThe 6 most common faults
01
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.
02
F5 High Engineer
Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.
Full guide for F503
F9 High Engineer
Your boiler is struggling to accurately detect how much water is inside the system, which is preventing it from safely starting your heating or hot water.
Full guide for F904
F12 High Engineer
Your boiler's electronic control panel is experiencing a communication error, which means it cannot process your settings or display status updates correctly.
Full guide for F1205
F13 High Engineer
Your boiler's internal computer is experiencing a communication error, meaning it can no longer safely control the heating or hot water functions.
Full guide for F1306
F17 High Engineer
Your boiler is struggling because the electricity supply it is receiving is not strong enough to power it correctly.
Full guide for F17on the Saunier Duval Isofast
Filling loopPressure sensorExpansion vesselOverheat thermostatCentral heating pumpDiverter 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