BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has reached an unsafe temperature and needs to cool down before it can restart.
Why it happens: This fault occurs when the water inside the boiler gets too hot, triggering a safety switch to prevent damage. The most common reason is that the water isn't moving through the system quickly enough, often due to a pump failure or a blockage in the pipework. It can also happen if the sensors that monitor temperature become faulty and send the wrong information to the boiler control board.
Check that your radiator valves are fully open to allow water to flow.Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).Press the 'Reset' button (flame symbol with a cross) once the boiler has cooled down.
Your boiler has detected that there is not enough water pressure in the system to operate safely.
Why it happens: This fault occurs when the boiler's internal sensors detect that the system pressure has dropped too low for it to heat up safely. The most common reason is a slow leak in the radiators or pipework, but it can also be triggered if the circulation pump has failed and is no longer moving water through the heat exchanger.
Check the pressure gauge on the front of the boiler or under the casing.Locate the external filling loop (braided silver hose) usually found beneath the boiler.Slowly open the valves until the pressure gauge reaches between 1.0 and 1.5 bar, then close them tightly.
Your boiler is struggling to circulate water properly, often because the system pressure is too low or there is trapped air preventing the heat from moving away from the boiler.
Why it happens: This fault occurs when the boiler detects a massive temperature difference between the water going out to your radiators and the water coming back, suggesting the heat isn't moving. The most common reasons are that the system doesn't have enough water pressure to move or there is a large pocket of air trapped inside the internal pump. In some cases, the pump itself has reached the end of its life and can no longer push the water around your home effectively.
Check the pressure gauge and top up to 1.5 bar using the filling loop if it is too lowBleed your radiators to remove any trapped air from the systemEnsure all radiator valves are fully open and not stuck
Your boiler is overheating because water isn't moving through the system quickly enough to take the heat away.
Why it happens: This fault occurs when the boiler's internal sensors detect that the temperature is rising far too quickly, indicating that the heat isn't being carried away from the heat exchanger. The most common reason is a failing or stuck circulation pump, though it can also be triggered by a significant air lock or a blockage in the pipework preventing water flow. Essentially, the boiler is protecting itself from damage by shutting down because the water inside it is standing still and boiling.
Check the pressure gauge and top up the system to 1.5 bar if it is too lowEnsure all radiator valves are fully openBleed your radiators and the pump to remove trapped air
Your boiler has detected that the water pressure has dropped too low or there is a problem with the internal sensor that monitors it, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler's internal safety mechanism detects that there isn't enough water pressure to operate safely. The most common reason is a physical leak in the system or air trapped in the radiators, but it can also be caused by a build-up of debris clogging the sensor or a loose electrical connection to the pressure switch.
Check the central heating pressure gauge; it should ideally be between 1.0 and 1.5 bar.If the pressure is below 1.0 bar, use the external filling loop to top up the system until the gauge reaches the correct level.Restart the boiler to see if the fault clears.
Your boiler has failed to ignite, which means it cannot light the gas flame required to provide heat or hot water.
Why it happens: This fault occurs when the boiler attempts to start up but fails to detect a flame after several attempts. The most common reasons are a lack of gas reaching the unit, worn-out spark electrodes that can no longer create a flame, or a fault within the internal gas valve that prevents fuel from entering the burner.
Check if other gas appliances in your home, such as a hob, are working to ensure gas is reaching the property.If you have a pre-payment meter, check that you have remaining credit.Ensure your gas stopcock is fully open.Press the reset button (indicated by a flame with a line through it) for one second.
on the Vaillant ecoMAX 635
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