BoilerFaultCodesThe 6 most common faults
Your boiler has sensed it is getting too hot and has shut itself down as a safety precaution.
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 your radiator valves are open to allow water to circulate.Ensure there is enough water pressure in the system (typically 1 to 1.5 bar).Wait for the boiler to cool down and press the reset button (flame symbol with a cross).
Your boiler has detected that there is not enough water inside the system to operate safely, forcing it to shut down to prevent damage.
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.
Check the pressure gauge on the front of the boiler; it should ideally be between 1 and 1.5 bar.Locate the filling loop (usually two braided hoses or blue handles) underneath the boiler.Slowly open the valves to let water into the system until the gauge reaches 1.2 bar, then ensure they are closed tightly.Press the 'Flame/Cross' button to reset the boiler.
Your boiler is heating up too quickly because the water isn't moving through the system properly, often caused by trapped air or a stuck pump.
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 that your boiler pressure is between 1.0 and 1.5 bar on the pressure gaugeBleed all radiators to remove trapped air from the systemEnsure all radiator valves and isolation valves under the boiler are fully openTry resetting the boiler using the button with the flame/cross symbol
Your boiler is failing to light the flame after three attempts, which is usually caused by a gas supply issue or a component failure in the ignition system.
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 if other gas appliances, like a cooker, are working to ensure gas is reaching your home.Check if your prepaid gas meter (if applicable) has credit.Try resetting the boiler by pressing the flame symbol button or the reset button on the front panel.
Your boiler has lost its flame while running and cannot manage to relight itself, meaning it has stopped providing heating and hot water.
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 other gas appliances like your hob are working.If you have a prepaid gas meter, verify that you have sufficient credit.Press the reset button (represented by a flame symbol with a cross) for one second.
Your boiler has stopped working because it cannot accurately measure the temperature of the water leaving the unit due to a sensor or wiring fault.
Why it happens: This fault occurs when the boiler's main control board loses communication with the temperature sensor that monitors the water leaving the unit. The most common reason is that the sensor itself has failed due to age or internal wear, though it can also be caused by loose wiring or water damage to the electrical connections inside the casing.
Full guide for F.00on the Vaillant Turbomax Plus 828
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