BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure is too low for it to operate safely.
Why it happens: This fault occurs when the internal water pressure drops so low that the boiler can no longer safely transfer heat from the burner to the water. The most common reasons are a slow leak hidden somewhere in your pipework or radiators, or the pressure being accidentally released through a safety valve.
Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.Slowly open the valve(s) until you hear water entering the system.Watch the pressure gauge and close the valves once the needle reaches between 1.0 and 1.5 bar.
Your boiler has detected that water isn't moving through the system fast enough, causing a large temperature difference that has triggered a safety shutdown.
Why it happens: This fault occurs when the water inside your boiler heats up much faster than it can be moved away into your radiators. The most common reason is a failing internal pump or a buildup of debris like sludge that is physically blocking the flow of water. When the boiler detects that the outgoing water is significantly hotter than the returning water, it shuts down to prevent the unit from overheating.
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barRestart the boiler using the reset button
Your boiler is overheating too quickly because water isn't flowing through the system correctly, often due to low pressure or trapped air.
Why it happens: This fault occurs when the boiler's internal sensors detect that the temperature is rising too rapidly, which usually means heat isn't being carried away from the heat exchanger. The most common reasons are that there isn't enough water pressure to move the heat, air bubbles are trapped in the pump, or the pump itself has seized and stopped circulating water. Basically, the boiler is producing heat faster than the system can distribute it to your radiators.
Check the pressure gauge and top up the water to 1.5 bar if it is too lowBleed any air from your radiators using a radiator keyEnsure all radiator valves and isolation valves are fully open
Your boiler is failing to light the gas flame, meaning it cannot provide heat or hot water.
Why it happens: This fault occurs when the boiler tries to light the gas flame but doesn't detect one after several attempts. The most common reason is a disruption in the gas supply, such as an isolated meter or a blockage, though it can also be caused by worn-out spark components that are no longer strong enough to ignite the fuel.
Check if your gas emergency control valve (usually by the meter) is in the 'on' position.If using a prepayment meter, ensure you have sufficient credit and the valve hasn't locked.Try resetting the boiler by pressing the flame symbol with a cross through it for five seconds.
Your boiler has lost its flame while running and cannot manage to restart itself, likely due to a temporary interruption in the gas supply or a component failure.
Why it happens: This fault occurs when the boiler’s flame is lost while it is already running, rather than failing to start initially. It is usually caused by a temporary drop in gas pressure, a build-up of debris on the ignition components, or an electrical fault that prevents the boiler from detecting its own flame. When the system can no longer confirm a stable flame is present, it shuts down as a safety precaution.
Check if other gas appliances, like a hob, are working to ensure gas is reaching the property.If you have a pre-payment meter, check that you have sufficient credit.Press the reset button (indicated by a flame symbol with a cross) for one second to see if the boiler restarts.
Your boiler has stopped working because it cannot detect the necessary change in pressure when the pump starts up.
Why it happens: This fault occurs when the boiler's internal computer fails to see a small 'jump' in water pressure that should happen every time the pump starts spinning. The most common reason is that the pressure sensor has become blocked with debris or the pump itself has seized, preventing the system from confirming it is safe to ignite. It can also happen if the system pressure is simply too low for the pump to move the water effectively.
Check the pressure gauge and top up the water to 1.5 bar if it is lowReset the boiler by holding the flame/cross button for five secondsBleed your radiators to ensure there is no trapped air in the system
on the Vaillant ecoTEC exclusive
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