BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because the water pressure 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 working level, causing the boiler to shut down to prevent internal damage. 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 (usually two silver braided hoses) underneath the boilerSlowly open both taps on the filling loop until you hear water movingClose both taps tightly once the pressure gauge on the screen reaches 1.5 bar
Your boiler has shut itself down because it has detected that it is getting far too hot, usually due to a lack of water flow or a circulation blockage.
Why it happens: This fault occurs when the boiler's internal water temperature rises too quickly or exceeds a safe limit, triggering an emergency shutdown to prevent damage. The most common reasons are a seized pump failing to move water, a buildup of sludge blocking the heat exchanger, or air trapped within the system preventing proper circulation.
Check that your boiler pressure is between 1.0 and 1.5 bar on the digital display or gauge.Ensure all radiator valves are fully open to allow water to circulate.Try resetting the boiler by pressing the button with the flame symbol 'X' crossed out.
Your boiler has shut down because water isn't moving through the system quickly enough, causing it to overheat too rapidly.
Why it happens: This fault occurs when the boiler's internal sensors detect that water is heating up too quickly or that there is a massive temperature difference between the pipes going out and coming back. The most common reason is a blockage or a failing pump preventing the water from circulating through your radiators. It essentially means the heat being generated has nowhere to go, so the boiler shuts down to prevent internal damage from overheating.
Check that all radiator valves (TRVs) are turned to the highest settingEnsure the boiler pressure is between 1.0 and 1.5 bar on the digital displayCheck for recent air locks and bleed your radiators if they have cold spots
Your boiler has shut down because it is heating up much faster than it can circulate the water, usually due to a blockage or a pump issue.
Why it happens: This fault occurs when the boiler detects that the water inside the unit is getting too hot, too quickly, suggesting the heat isn't being carried away to your radiators. It is usually caused by a pump failure, a blockage caused by sludge buildup, or air trapped within the system preventing proper water flow. Effectively, the boiler is forced to shut down to prevent the internal components from overheating and sustaining damage.
Check that all radiator valves (TRVs) are fully open to allow water to flow.Check the boiler pressure gauge and top it up to 1.5 bar if it is too low.Try resetting the boiler using the flame/reset button.
Your boiler is unable to light the gas to produce heat, meaning your heating and hot water will not work.
Why it happens: This fault occurs when the boiler attempts to ignite three times but fails to establish a steady flame. The most common reasons include an interrupted gas supply, a worn-out ignition spark plug, or a component failure that prevents the gas valve from opening correctly.
Check if your gas meter has credit (if using a pre-payment meter).Ensure your emergency gas lever or external gas valves are fully open.Restart the boiler by pressing the reset button (the flame symbol with a cross) for one second.
Your boiler has lost its flame while running and is unable to automatically restart, likely because the gas flow is being interrupted.
Why it happens: This fault occurs when your boiler successfully starts but lose its flame while it is trying to heat your home or water. The most common reasons are a temporary drop in gas pressure, a build-up of debris on the burner, or a faulty internal component that can no longer detect the flame. If it happens during very cold weather, it may also be caused by a frozen external pipe blocking the system's drainage.
Check if other gas appliances in your home, like a hob, are working correctly.If you have a prepayment meter, ensure you have sufficient credit.During freezing weather, check if the white condensate pipe leading outside is frozen and thaw it with warm water.Press the Reset button (indicated by a flame symbol with a cross) for one second.
on the Vaillant ecoTEC Pro
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