BoilerFaultCodesThe 6 most common faults
Your boiler is struggling to breathe properly because the exhaust pipe outside might be blocked or the gas is not burning correctly, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler's internal sensors detect that the gas isn't burning cleanly or safely. The most common reason is a physical obstruction in the flue pipe outside, but it can also be caused by an incorrect mixture of gas and air during the ignition process. To protect you from carbon monoxide, the system automatically shuts down when it detects these poor combustion levels.
Check the external flue terminal (the exhaust pipe on your outside wall) for obstructions like bird nests, leaves, or debris.Carefully remove any visible blockages from the end of the pipe.Reset the boiler by turning the selector switch to 'R' for at least five seconds.
Your boiler has stopped working because the water pressure in the system has dropped too low.
Why it happens: This fault occurs when the water pressure inside your heating system drops below the minimum level required for the boiler to operate safely. The most common reasons are a slow leak from a pipe or radiator valve, or air being bled from the radiators without the system being topped back up with water.
Locate the filling loop (usually a silver braided hose with small lever valves under the boiler).Slowly open both lever valves to allow water into the system.Watch the pressure gauge and close both valves firmly once it reaches between 1.0 and 1.5 bar.
Your boiler is not firing up properly, which means it cannot light the gas to provide heat or hot water.
Why it happens: This fault occurs when the boiler tries to light the gas but fails to detect a flame after several attempts. The most common reasons include a worn-out ignition spark, a faulty gas valve that isn't letting fuel through, or an issue with the electrical circuit board that controls the starting sequence.
Check if your gas supply is active by testing another gas appliance like a hob.Ensure your credit is topped up if you have a pre-payment gas meter.Press and hold the 'R' or 'Reset' button for at least 2 seconds.
Your boiler has lost its fine-tuning and needs to undergo a self-calibration process to ensure the gas and air are burning efficiently.
Why it happens: This fault occurs when the boiler's internal computer loses track of the precise balance between gas and air needed for a clean flame. It often happens after a power cut, a sudden change in gas pressure, or if the internal components have worn slightly over time, causing the system to lose its 'fine-tuning'.
Press and hold the 'R' (Reset) button for at least 2 seconds.Wait for the boiler to complete its automatic calibration sequence, which may take several minutes.Ensure all radiator valves are open to allow the boiler to disperse heat during the process.
Your boiler is struggling to move water around the system effectively, which is causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler detects that water is not flowing through the internal pipework quickly enough to carry heat away. The most common reasons are a pump that has seized up or a build-up of sludge and debris that is physically blocking the narrow passages inside the main heat exchanger.
Check that the boiler pressure is between 1 and 1.5 bar on the pressure gauge.Ensure all radiator valves and internal stop taps are fully open.Gently bleed your radiators to remove any trapped air pockets.
Your boiler started to heat up but the flame went out unexpectedly while it was running.
Why it happens: This fault occurs when the boiler's control system loses the signal that a flame is present while it is already firing. The most common reason is that the flame sensing electrode has become worn or dirty and can no longer detect the heat, although it can also be caused by an unstable gas supply or a failing component preventing the gas from flowing correctly.
Check that your gas emergency control valve is fully openIf you have a prepaid gas meter, check that you have sufficient creditAttempt to reset the boiler by turning the selector switch to 'R' for five seconds
on the Baxi Duo-tec Combi
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 Baxi 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