BoilerFaultCodesThe 6 most common faults
Your boiler has detected that water is not circulating correctly through the system, preventing it from heating up safely.
Why it happens: This fault occurs when the boiler's internal sensors detect that water isn't moving through the unit quickly enough to carry heat away safely. The most common reasons are a seized circulation pump, a blockage in the pipework, or a failure in the switch that monitors water flow. Because the heat cannot exit the boiler, the system shuts down to prevent the internal components from overheating.
Check that your boiler pressure is between 1.0 and 1.5 bar on the gaugeEnsure all radiator valves and isolation valves under the boiler are fully openBleed your radiators to remove any trapped air pocketsReset the boiler by turning the selector switch to 'R' for five seconds
Your boiler is struggling to move water around the system before it starts the ignition process, which is preventing it from firing up.
Why it happens: This fault occurs when the boiler's internal sensors detect that water isn't moving through the unit quickly enough just before the burner ignites. The most common reason is a failing or seized circulation pump, though it can also be triggered by a build-up of sludge or debris blocking the pathways within the heat exchanger.
Check that the boiler pressure is between 1.0 and 1.5 bar on the gaugeIf pressure is low, use the filling loop to top it upEnsure all radiator valves and isolation valves under the boiler are fully openRestart the boiler using the reset button
Your boiler has shut down because it has detected it is getting too hot too quickly, usually caused by poor water circulation or trapped air.
Why it happens: This fault occurs when the water inside the boiler heats up so quickly that the safety sensors shut the system down to prevent damage. The most common cause is a blockage or a failed pump preventing water from moving through the pipes, but it can also be triggered by large air bubbles trapped within the unit.
Check that your radiator valves are open and not turned to the off position.Carefully bleed your radiators to remove any trapped air from the system.Restart the boiler by turning the selector switch to the reset position for five seconds.
Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Why it happens: This fault occurs when the water pressure inside your heating system drops below a safe level, causing the boiler to shut down to prevent internal damage. The most common reasons are a slow leak from a radiator valve or pipework, or air being released from the system through the radiators or an internal auto-air vent.
Locate the filling loop (usually a silver braided hose with two blue-handled valves) underneath the boiler.Slowly open both valves until you hear water entering the system.Watch the pressure gauge and close both valves firmly once the needle reaches between 1.0 and 1.5 bar.
Your boiler has detected that hot water isn't moving through the system properly, which is likely caused by a blockage or a pump issue.
Why it happens: This fault occurs when the internal sensors detect that water is getting too hot too quickly because it isn't moving away from the heat exchanger. The most common reasons are a seized circulation pump, a blockage caused by sludge buildup, or an air lock trapped within the pipework.
Check if all radiator valves are openEnsure your boiler pressure is between 1.0 and 1.5 barTry resetting the boiler to see if the error clears
Your boiler is struggling to keep a steady flame alight and has failed to automatically adjust itself after several attempts.
Why it happens: This fault occurs when the boiler's internal computer tries to adjust the gas and air mix to create a steady flame, but the flame keeps flickering or blowing out. The most common reason is that the sensing probe has become dirty or worn out, meaning the boiler can no longer 'see' the flame even if it is actually lit. It can also be caused by a faulty gas valve or an unstable gas supply that prevents the boiler from completing its automatic calibration.
Check that your gas meter has credit (if on a pre-payment meter)Ensure other gas appliances in your home, like a hob, are working correctlyPress the Reset button for five seconds to restart the calibration process
on the Baxi 600 Combi (range)
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