BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has detected that water is failing to move through the system correctly, causing it to overheat.
Why it happens: This fault occurs when water cannot travel through the boiler fast enough to carry heat away, causing the internal components to get too hot. This is usually caused by a failing pump that can no longer push the water, or a buildup of sludge and debris creating a blockage within the system.
Check that all radiator valves are open and not turned off.Ensure your system water pressure is within the green zone (typically 1.0 to 1.5 bar).Press the reset button to see if the fault clears after the system has cooled down.
Your boiler has sensed it is getting too hot and has shut itself down as a safety precaution.
Why it happens: This fault happens when water inside the boiler reaches a temperature that is no longer safe, triggering an automatic shutdown to prevent damage. It is usually caused by a pump failure preventing water from moving away from the heat exchanger, or a build-up of sludge and debris that blocks the flow. In some cases, a faulty sensor might also incorrectly signal that the system is overheating.
Check that all radiator valves are fully open to allow water to flow.Ensure the system pressure is between 1 and 1.5 bar on the pressure gauge.Turn the control knob to the 'Reset' position for five seconds and then back to 'On'.
Your boiler thinks a flame is present when it shouldn't be, causing the system to shut down for safety purposes.
Why it happens: This fault occurs when the boiler's control system detects an electrical signal from the burner even though it hasn't commanded the gas to ignite yet. The most common reasons are a contaminated flame sensor sending a rogue signal, a gas valve that is leaking slightly, or a fault within the main circuit board that is misinterpreting electrical noise as a flame.
Full guide for False Flame FaultYour boiler has experienced an internal computer failure and has shut down for safety, meaning it will no longer provide heating or hot water.
Why it happens: This fault occurs when the boiler's internal control center suffers a critical hardware failure or a short circuit on its circuitry. The most common reasons are electrical surges, general wear and tear of the components over time, or moisture ingress from a slow internal leak that has reached the sensitive electronic board.
Full guide for PCB FaultYour boiler has stopped working because it cannot confirm that harmful exhaust gases are being safely cleared through the flue pipe.
Why it happens: This fault occurs when the boiler's internal safety sensor cannot detect a steady flow of air through the system, meaning it isn't sure that exhaust fumes are leaving the house safely. The most common reasons are a failing fan that isn't spinning fast enough, a blockage in the flue pipe, or a perished rubber tube that has stopped the sensor from reading air pressure correctly.
Full guide for Air Proving LOCKOUTYour boiler has stopped working because its internal safety switch is stuck and cannot confirm it is safe to start the burner.
Why it happens: This fault occurs when the boiler's internal safety sensor, called the Air Pressure Switch, is stuck in the 'on' position before the fan has even started. The boiler refuses to fire up because it cannot confirm that harmful exhaust gases are being safely cleared through the flue. The most common reasons are a build-up of debris or moisture in the small sensing tubes, or a mechanical failure within the switch itself.
Full guide for APS Stuck Faulton the Potterton Performa SL HE 9 2
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 Potterton 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