BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure is too low for it to run safely.
Why it happens: This fault occurs when the water level inside your central heating system drops below the minimum level required to operate. The most common reason is a slow leak from a radiator valve or pipework, though it can also happen if you have recently bled your radiators and haven't topped the pressure back up.
Locate the filling loop (the silver braided hose beneath the boiler).Slowly open the one or two taps on the hose until you hear water flowing.Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.
Your boiler has switched itself off because it has detected that the water inside is getting too hot.
Why it happens: This fault occurs when the boiler's internal temperature rises too quickly or exceeds a safe limit, causing the system to shut down to prevent damage. The most common reasons are a lack of water flow caused by a faulty pump, a blockage in the central heating pipes, or air trapped within the heat exchanger.
Check that your radiator valves are fully openEnsure your central heating pump is running and not stuckPress the reset button (flame symbol with a cross) once the boiler has cooled down
Your boiler is struggling to move water around the system properly, causing it to overheat and shut down for safety.
Why it happens: This fault occurs when the boiler detects that water isn't circulating through the pipes and radiators fast enough to carry heat away from the burner. The most common reasons are a seized internal pump or a blockage in the system, such as a buildup of sludge or a clogged heating filter, which prevents the water from moving freely. Because the heat cannot escape, the boiler reaches a dangerous temperature and shuts down to prevent internal damage.
Check that the boiler pressure is between 1 and 1.5 bar on the displayEnsure all radiator valves and the boiler service valves are fully openGently bleed air from your radiators using a radiator key
Your boiler is heating up too quickly because water isn't flowing through the system properly, causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler's internal sensors detect that the water temperature is rising much faster than the system can handle. The most common reason is a lack of circulation, often caused by a failing pump or a blockage in the pipework that prevents the heat from being moved away from the boiler quickly enough. It can also be triggered if there is air trapped in the heat exchanger or if the system water pressure has dropped too low.
Check that the boiler pressure is between 1.0 and 1.5 bar on the display.Ensure all radiator valves and isolation valves under the boiler are fully open.Bleed your radiators to remove any trapped air pockets.If pressure is low, use the filling loop to top it up.
Your boiler is not receiving gas or is unable to light a flame to provide heating and hot water.
Why it happens: This fault occurs when the boiler tries to light itself several times but fails to detect a flame, causing the system to shut down for safety. The most common reasons include a lack of gas reaching the unit, a blocked burner, or a component failure in the ignition system that prevents the initial spark. It can also be triggered by external issues like a shut-off gas meter or a temporary interruption in your local gas supply.
Check if other gas appliances, like your hob, are working to ensure gas is reaching the property.If you have a prepaid meter, check that you have enough credit.Press the 'Flame' or 'Reset' button on the boiler controls for five seconds.
Your boiler has lost its flame while running and is now unable to relight itself to provide heat or hot water.
Why it happens: This fault occurs when your boiler successfully starts but cannot maintain a steady flame while it is running, leading the system to shut down for safety. The most common reasons include a temporary interruption in your gas supply, a build-up of carbon on the ignition sensors, or a component failure that prevents the correct mix of gas and air from reaching the burner.
Check if other gas appliances in your home, like a hob, are working to ensure gas is reaching the propertyCheck if your gas meter has credit (if using a pre-payment meter)If it is freezing outside, check if the white plastic condensate pipe leading outdoors is frozen and thaw it with warm waterPress the reset button (indicated by a flame symbol with a cross) for one second
on the Vaillant ecoTEC
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 Vaillant 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