BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because it has detected it is getting too hot, usually caused by a lack of water flow or a blockage in the system.
Why it happens: This fault occurs when the boiler's internal heat exchanger reaches an unsafe temperature, forcing the system to shut down to prevent damage. The most common reasons are a lack of water flow caused by a broken internal pump, or a blockage in the pipes resulting from a build-up of sludge and debris.
Check if your radiator valves are openCheck the boiler pressure gauge and top up if it is below 1.0 barPress the reset button (flame symbol) to see if the fault clears
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 water pressure drops below the safe operating limit, causing the boiler to shut down to prevent damage to the heat exchanger. The most common reasons for this are a leak somewhere in the central heating pipework or radiators, or simply that the system needs to be topped up after air was bled from the radiators.
Locate the filling loop valves underneath your boilerSlowly open both valves to let water into the systemClose both valves tightly once the pressure gauge reaches 1.5 bar
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
Your boiler has shut down because it is heating up much faster than it should, which usually means the water isn't moving through the system properly.
Why it happens: This fault occurs when the boiler detects that the internal temperature is rising too quickly, triggering a safety shutdown to prevent overheating. It is usually caused by a blockage in the pipes or a failing pump that prevents hot water from leaving the boiler quickly enough. Essentially, the heat can't escape into your radiators, so it builds up inside the unit almost instantly.
Check that your boiler pressure is between 1.0 and 1.5 bar on the digital displayEnsure all radiator valves are fully open to allow water to flowPress the reset button (flame symbol with a cross) to see if the fault clears
Your boiler has failed to ignite and has locked itself out for safety, meaning you currently have no heating or hot water.
Why it happens: This fault occurs when the boiler tries to light the flame but fails to detect one after several attempts. The most common reasons are a lack of gas reaching the unit, worn-out spark components that can't create a flame, or a faulty internal valve that isn't releasing fuel correctly.
Check if your home has gas by testing another gas appliance like a hob.Ensure your gas meter has credit and the isolation valve is open.Wait 5 minutes and then press the 'Reset' button (indicated by a flame with a cross through it).
Your boiler was working but has suddenly turned off because the flame has gone out and it cannot manage to relight itself.
Why it happens: This fault occurs when the boiler successfully starts but lose its flame while it is trying to heat your home or water. The most common reasons are a temporary interruption in the gas supply, a blocked flue, or internal components like electrodes becoming too dirty to detect the flame. It can also be caused by a buildup of debris in the condensate trap preventing the burner from staying lit.
Check if other gas appliances, like a hob, are still working to ensure your gas supply hasn't been interrupted.If it is freezing outside, check the condensate pipe (the plastic pipe leading outside) for ice and defrost it with warm water.Press the reset button (a flame symbol with a cross through it) for five seconds to see if the boiler restarts.
on the Vaillant ecoTEC Pro / 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