BoilerFaultCodesThe 4 most common faults
01
Red light F ON High DIY-safe
Your boiler has detected a safety issue and has automatically switched itself off to prevent damage or dangerous operation.
What to try first
Locate the reset button labeled 'G' on the front control panel.Press and hold the button firmly for a few seconds to see if the boiler restarts.If the boiler fires up, monitor it for a few hours to ensure the light does not return.
02
Low water pressure LED High DIY-safe
Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.
What to try first
Locate the two small taps (filling loop) usually found underneath the boilerOpen the taps slowly to let fresh water into your heating system until the pressure gauge needle reaches the green zone (typically 1.0 to 1.5 bar)Close both taps firmly to prevent over-pressurisation
03
Mains LEDs High Engineer
Your boiler has lost all power and is completely unresponsive, likely due to an internal electrical protection component failing.
Full guide for Mains LEDs04
Lockout High Engineer
Your boiler has stopped working because it cannot successfully start or maintain a flame, so it has safely shut itself down to prevent gas from building up.
Full guide for Lockouton the Saunier Duval SD 30e
Overheat thermostatLimit switchIgnition leadPressure relief valvePressure switchExpansion vessel
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