BoilerFaultCodesThe 6 most common faults
Your boiler is unable to safely light the flame needed to provide heating and hot water.
Why it happens: This fault occurs when the boiler attempts to ignite several times but fails to establish or detect a steady flame. It is often caused by a lack of gas reaching the burner, worn-out spark electrodes, or a faulty sensor that cannot confirm the flame is safely lit.
Check that your gas prepayment meter has credit (if applicable)Verify that other gas appliances, such as a hob, are working correctlyPress and hold the reset button (flame symbol with a cross) for five seconds
Your boiler is unable to safely light the flame needed to provide heating and hot water.
Why it happens: This fault occurs when the boiler attempts to ignite several times but fails to establish or detect a steady flame. It is often caused by a lack of gas reaching the burner, worn-out spark electrodes, or a faulty sensor that cannot confirm the flame is safely lit.
Check that your gas prepayment meter has credit (if applicable)Verify that other gas appliances, such as a hob, are working correctlyPress and hold the reset button (flame symbol with a cross) for five seconds
Your boiler has switched itself off because it has reached an unsafe temperature to protect the internal components from damage.
Why it happens: This fault occurs when the water inside the boiler gets too hot, causing a safety sensor to trip and shut the system down. The most common reasons are a pump failure that stops water from circulating or a buildup of sludge and debris that restricts flow through the heat exchanger.
Check that all radiator valves are fully open to allow water to circulateEnsure the boiler pressure gauge is between 1 and 1.5 barPress the reset button once to see if the fault clears after the system has cooled down
Your boiler has detected an issue with the sensor that monitors water pressure, which means it cannot verify if there is enough water to run safely.
Why it happens: This fault occurs when the boiler's internal computer loses communication with the sensor that measures water levels within the system. The most common reason is that the sensor has suffered an electrical failure or has become blocked by debris and sludge, preventing it from sending a reliable reading. Because the boiler cannot confirm there is enough water to operate safely without overheating, it shuts down as a precaution.
Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar.If the pressure is too low, use the filling loop underneath the boiler to carefully top it up.Press the reset button (usually marked with a flame or 'reset') once the pressure is corrected.
Your boiler has sensed that water is not moving through the system properly, which is causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler detects a significant temperature difference between the water leaving the unit and the water returning to it, suggesting a blockage or pump failure. The most common reason is a seized or failing circulation pump that can no longer push water through the radiators and pipework. Additionally, a build-up of sludge or limescale within the heat exchanger can restrict flow enough to trigger this safety shutdown.
Check that the boiler pressure is between 1 and 1.5 bar on the displayEnsure all radiator valves are fully openListen for air in the system and bleed radiators if necessary
Your boiler has detected that the water is getting hot too quickly or the temperature difference between pipes is too wide, often caused by poor water flow.
Why it happens: This fault occurs when the boiler's internal sensors detect that heat isn't being carried away from the burner fast enough, causing the unit to overheat rapidly. The most common reason is a blockage in the system or a pump failure, which prevents water from circulating through the radiators as it should.
Check that all radiator valves and isolation valves are fully openEnsure there is enough water pressure by checking the boiler gauge (target 1.5 bar)Bleed all radiators to remove trapped air that might be blocking flowReset the boiler to see if the fault clears
on the Saunier Duval Isotwin F30E
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