BoilerFaultCodes
The most common Vaillant faults
Your boiler has stopped working because the water pressure is too low, and it has shut down to prevent damage.
Why it happens: This fault occurs when the system loses too much water, causing the internal pressure to drop below a safe operating level. The most common reasons are a slow leak somewhere in your radiators or pipework, or a component like the pressure relief valve failing and letting water escape. Without enough water pressure, the boiler cannot circulate heat safely and shuts down to prevent the internal components from overheating.
Check the pressure gauge on the front of the boiler or under the casing.Locate the external filling loop (usually a silver flexible hose with small taps).Open the taps slowly until the pressure needle reaches between 1.0 and 1.5 bar, then close them tightly.Press the reset button on the boiler control panel.
Your boiler has switched itself off because it has become too hot internally and needs time to cool down before it can be used again.
Why it happens: This fault occurs when the water inside the heat exchanger reaches an unsafe temperature, triggering a safety shut-off. The most common reasons are a pump failure that prevents water from moving, a blockage in the pipework, or a Build-up of limescale and sludge. Essentially, the boiler is generating heat but has no way to move it away into your radiators or hot water tank.
Check that all radiator valves are fully open to allow water to flow.Wait 30 minutes for the unit to cool down completely.Press the reset button (indicated by a flame with a cross through it) on the front control panel.
Your boiler has detected that there is not enough water pressure in the system to operate safely, causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler's internal sensor detects that the system pressure has dropped too low for the unit to heat water safely. The most common reason is a slow leak somewhere in your radiator network or a faulty expansion vessel that can no longer regulate pressure correctly. Without enough water to circulate, the boiler shuts down to prevent the heat exchanger from overheating or sustaining permanent damage.
Locate the silver braided filling loop hoses underneath or near the boiler.Open the two small valves on the filling loop to allow water into the system.Watch the pressure gauge on the front of the boiler until it reaches between 1.0 and 1.5 bar, then close both valves tightly.
Your boiler is struggling to move water around the system, often because the water level is too low or something is blocking the flow.
Why it happens: This fault occurs when the boiler detects a sudden, sharp rise in temperature because the heated water isn't moving away from the burner quickly enough. The most common reasons are a lack of water pressure in the system, a pump that has seized up, or a physical blockage like sludge or debris preventing circulation.
Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar.If the pressure is low, use the filling loop valves underneath the boiler to top it up.Ensure all radiator valves are fully open to allow water to flow freely.
Your boiler is trying to start up but cannot light the gas flame required to provide heat or hot water.
Why it happens: This fault occurs when the boiler attempts to ignite several times but fails to establish a consistent flame. The most common reasons include a lack of gas reaching the burner, worn-out spark electrodes that can no longer create a flame, or a faulty gas valve that isn't opening correctly. This effectively acts as a safety shutdown to prevent the boiler from trying to operate without a fire.
Check that your gas emergency control valve is open and you have credit on your meter.Test another gas appliance, like a hob, to ensure gas is reaching the property.Press the 'Flame' or 'Reset' button on the boiler control panel for one second.
Your boiler was working but the flame has gone out unexpectedly, often due to an interrupted gas supply or a blockage in the drainage pipe.
Why it happens: This fault occurs when the boiler successfully lights but the flame is lost unexpectedly during operation. The most common reasons are an interruption in the gas supply, a blocked condensate drainage pipe, or a build-up of carbon on the sensing electrodes which prevents the boiler from 'seeing' the flame.
Check that your gas emergency control valve is open and you have credit on your meter.If it is freezing outside, check if the plastic condensate pipe leading outdoors is frozen and thaw it with warm water.Press the 'Flame/Cross' reset button for one second to see if the boiler restarts.
Your boiler has detected that water isn't moving through the internal pipes correctly, causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler's internal sensors detect a massive temperature difference between the water entering and leaving the unit, usually because the water has stopped moving. The most common reason is a failed internal pump or a heavy buildup of sludge and debris blocking the main heat exchanger. Without proper flow, the water inside the unit reaches a boiling point almost instantly, forcing the system to shut down for safety.
Check that all radiator valves are fully openEnsure the internal boiler valves underneath the unit are in the open positionCheck if the pump is making a humming sound, which may indicate it is stuck
Your boiler has switched itself off because the water temperature inside it is rising much faster than the water temperature being sent to your radiators, usually due to a circulation blockage.
Why it happens: This fault occurs when the internal water temperature rises too quickly because the heated water cannot leave the boiler fast enough to reach your radiators. The most common reason is a pump failure or a significant blockage in the system, such as a build-up of sludge or debris, which prevents the water from circulating properly.
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 if any valves underneath the boiler have been accidentally closed