BoilerFaultCodes
The most common Vaillant faults
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 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 struggling to light its flame, meaning your heating and hot water will likely stop working soon.
Why it happens: This fault occurs when the boiler's control system attempts to start the heating cycle but fails to successfully establish or detect a flame. The most common reasons include a worn-out ignition spark, a faulty gas valve not releasing fuel, or the internal sensors becoming too dirty to 'see' the flame once it has lit.
Check that your gas pre-payment meter (if you have one) has creditEnsure your gas stopcock is fully openPress the Reset button (backwards arrow or 'Flame' icon) for five seconds
Your boiler has failed to ignite and is not producing heat, which is usually due to an interrupted gas supply or a problem with the internal spark system.
Why it happens: This fault occurs when the boiler tries to light the flame but fails to detect one, causing the system to shut down for safety. The most common reasons are a temporary interruption in the gas supply, a build-up of carbon on the ignition probes, or a component failure that prevents the spark from starting the combustion process.
Check if other gas appliances, like a cooker, are working to confirm gas is reaching the propertyEnsure your gas meter has credit if you use a pre-payment accountPress the reset button (indicated by a flame with a line through it) and hold for two seconds
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
Affiliate links. We may earn a commission at no extra cost to you.