BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has reached an unsafe temperature and needs to cool down.
Why it happens: This fault occurs when the boiler's internal sensors detect that the water temperature has risen too quickly or exceeded a safe limit, triggering an emergency shutdown to prevent damage. The most common cause is a lack of water circulation, often due to a seized pump, a blockage in the pipework, or air trapped within the heat exchanger.
Ensure all radiator valves are fully open to allow water to flow.Check the boiler pressure gauge and top up to 1.5 bar if it is too low.Press the 'Flame' or 'Reset' button and hold for one second to restart the system.
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 internal pressure sensor detects that there isn't enough water in the system to circulate heat safely. The most common reasons are a slow leak hidden somewhere in your pipework or radiators, or the need to bleed air out of the system after a period of heavy use.
Locate the filling loop (usually two braided silver hoses or blue levers) underneath the boiler.Slowly open both valves until you hear water flowing and the pressure gauge reaches between 1.0 and 1.5 bar.Close both valves tightly and restart the boiler to clear the fault code.
Your boiler has shut down because there is a significant difference between the water temperature leaving the boiler and the temperature returning to it, usually caused by a circulation problem.
Why it happens: This fault occurs when water is leaving the boiler much hotter than when it returns, which suggests the water isn't moving through your radiators quickly enough. The most common reasons are a failing internal pump that can't push the water effectively or a blockage caused by a buildup of sludge and debris in the pipework.
Ensure all radiator valves (TRVs) are fully openCheck that the boiler pressure is between 1.0 and 1.5 barBleed your radiators to remove any trapped air
Your boiler has shut down because the internal water temperature is heating up much faster than it can be circulated away, usually meaning the water is stuck or the pump is struggling.
Why it happens: This fault occurs when the heat inside the boiler builds up too quickly because the heated water isn't moving through your radiators fast enough. The most common reasons are a failing circulation pump, a blockage in the system like sludge or limescale, or air trapped within the pipes.
Check that your boiler pressure is between 1.0 and 1.5 bar on the digital display.Ensure at least two or three radiator valves are turned fully open to allow water flow.Try resetting the boiler by pressing the flame symbol button to see if the fault clears.
Your boiler is trying to light itself to provide heat, but it is failing to create a flame, likely because it is not getting enough gas or the spark isn't catching.
Why it happens: This fault occurs when your boiler attempts to fire up but cannot detect a stable flame after several attempts. The most common reasons are a lack of gas reaching the unit, a worn-out spark plug that cannot ignite the fuel, or a internal sensor that is too dirty to 'see' the flame correctly.
Check that your gas emergency control valve is open and any prepay meters have credit.Verify if other gas appliances in your home, like a hob, are working correctly.If it is freezing outside, check if your condensate pipe (the plastic pipe leading outside) is frozen and thaw it with warm water.Press the Reset button (the flame symbol with a cross) for one second.
Your boiler was working but the flame went out and it cannot manage to light itself back up.
Why it happens: This fault occurs when the boiler is running normally but the flame suddenly disappears and the system fails to relight it. The most common reasons include a temporary interruption in the gas supply, a blocked flue vent, or a buildup of carbon on the ignition electrodes that prevents them from detecting the flame.
Check if other gas appliances (like a hob) are working to ensure gas is reaching the propertyCheck if your gas meter has credit and the isolation valve is openPress the 'Reset' button (indicated by a flame with a line through it) for three seconds
on the Vaillant ecoTEC plus
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