BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has become too hot internally, usually due to a lack of water flow or a blockage.
Why it happens: This fault occurs when the water inside the boiler reaches a temperature that is dangerously high, triggering a safety shut-off. This is usually caused by a pump failure, a blockage in the pipework, or a build-up of sludge that prevents heat from moving away from the internal heat exchanger. Effectively, the heat is being generated but has nowhere to go, causing the system to protect itself by locking out.
Check that your boiler pressure is between 1.0 and 1.5 barEnsure all radiator valves and isolation valves are fully openPress the reset button (usually a flame symbol with a cross through it) once the boiler has cooled down
Your boiler has detected that there is not enough water pressure or flow to operate safely and has shut down to prevent overheating.
Why it happens: This fault occurs when the boiler detects a critical lack of water or a failure in the circulation system, preventing it from moving heat away from the burner. The most common reason is a significant drop in system pressure, but it can also be triggered if the pump has seized or failed to start. Without sufficient water flow, the internal sensors shut the system down immediately to prevent the heat exchanger from melting or cracking.
Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar.If the pressure is too low, use the external filling loop valves to top up the system with water.Attempt to reset the boiler once the correct pressure is reached.
Your boiler has detected that the water isn't moving through the system correctly, causing it to heat up too quickly and shut down for safety.
Why it happens: This fault occurs when the boiler detects a massive temperature gap between the pipes leaving the unit and those returning, suggesting that water has stopped circulating. The most common reason is a failed or jammed internal pump, but it can also be caused by a severe blockage in the system or air being trapped within the heat exchanger.
Check if the central heating valves under the boiler are fully openCheck the boiler pressure gauge and top it up to 1.5 bar if it is too lowEnsure all radiator valves (TRVs) are turned to their highest setting
Your boiler is heating up too quickly because water isn't moving through the system properly, causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the internal sensors detect that the water inside the boiler is getting hot far too quickly, which usually means the water isn't flowing away from the heat exchanger. The most common reason is a failing pump that can't push the water around, though it can also be caused by a large air bubble trapped in the pipework blocking the path.
Check that all radiator valves and isolation valves are fully openBleed your radiators to remove any trapped air from the systemEnsure the boiler pressure is between 1.0 and 1.5 bar on the pressure gauge
Your boiler is failing to light because it isn't receiving gas or the ignition system isn't creating a spark.
Why it happens: This fault occurs when the boiler attempts to ignite several times but fails to establish a flame, leading the system to shut down for safety. The most common reasons are a lack of gas reaching the unit, a worn-out spark electrode that can no longer light the burner, or a faulty gas valve failing to open. In some cases, it can be caused by something as simple as a recent gas meter exchange or the gas supply being temporarily turned off.
Check that your gas meter has credit and the isolation valve is openCheck if other gas appliances, like a cooker, are workingPress the reset button (flame symbol with a cross) for one second
Your boiler has lost its flame while running and cannot restart, likely because it is not getting enough gas or the ignition system is failing.
Why it happens: This fault occurs when the boiler successfully starts but the flame goes out unexpectedly while it is trying to heat your home. The most common reasons are a temporary interruption in your gas supply, a buildup of carbon on the sensing probes, or a failing gas valve that can no longer maintain a steady flow. Essentially, the boiler's safety system steps in because it can no longer detect the heat of a consistent flame.
Check that your gas emergency control valve is in the 'on' positionIf you have a prepayment meter, ensure you have sufficient creditTry to reset the boiler by pressing the flame/cross button to see if it reignites
on the Vaillant Thermocompact
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