BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because the water inside it has become too hot, which is a safety precaution to prevent damage.
Why it happens: This fault occurs when the internal safety sensors detect that the water inside the boiler has reached a dangerously high temperature. It is usually caused by a failure in the pump preventing water from moving through the system or a blockage that is trapping heat within the heat exchanger.
Check that all radiator valves are fully openWait 30 minutes for the system to cool downPress the 'Reset' button on the front of the boiler
Your boiler has stopped working because the water pressure is too low for the system to run safely.
Why it happens: This fault occurs when the pressure of the water circulating inside your heating system drops below a safe working level, usually because of a small leak or after you have recently bled your radiators. Without enough water pressure, the boiler cannot safely move heat around your home, so it shuts down to prevent the internal components from overheating. While often fixed by topping up the system, persistent pressure loss may indicate a failing part like the expansion vessel.
Locate the filling loop (usually a silver flexible hose) under or near the boiler.Slowly open the one or two taps/levers on the filling loop until you hear water entering.Watch the pressure gauge and close the taps once the needle reaches between 1.0 and 1.5 bar.
Your boiler has experienced a power cut or was switched off while it was already showing a different fault, causing it to lock itself out for safety.
Why it happens: This fault occurs when the boiler loses electrical power—either due to a local power cut or someone switching off the fused spur—while it was already struggling with a different internal issue. Because the boiler was interrupted while trying to process an existing error, it enters a protective 'lockout' state to prevent you from accidentally running the system in an unsafe condition. It effectively gets stuck in a loop between the old error and the sudden loss of power.
Locate the selector switch on the front of the boilerTurn the switch to the 'R' (Reset) position and hold it for at least five secondsTurn the switch back to the 'On' position and wait to see if a different fault code appears
Your boiler is failing to light the gas burner, meaning it cannot provide heating or hot water until the issue is resolved.
Why it happens: This fault occurs when the boiler attempts to ignite but fails to establish or maintain a steady flame. The most common reasons include a lack of gas reaching the unit, a worn-out ignition spark, or a sensor that can no longer detect the flame once it is lit.
Check if your gas meter has credit and that the gas isolation valve is open.Try to reset the boiler by turning the selector switch to 'R' for five seconds and then back to the on position.In freezing temperatures, check if the external white plastic condensate pipe has frozen and needs thawing with warm water.
Your boiler's temperature sensor has encountered a technical glitch, meaning the system cannot accurately monitor the heat of the water it is pumping around your radiators.
Why it happens: This fault occurs when the internal thermistor, which monitors the temperature of the water leaving the boiler, sends an incorrect signal or stops communicating entirely. The most common reason is that the sensor has degraded over time due to constant heating and cooling, or the electrical wiring connecting it to the main control board has developed a loose connection.
Full guide for E20Your boiler has detected a problem with the sensor that monitors the temperature of the exhaust gases, causing the system to shut down for safety.
Why it happens: This fault occurs when the sensor responsible for monitoring the temperature of the exhaust gases sends an incorrect or missing signal to the boiler's control board. The most common reason is that the sensor has physically failed due to age, but it can also be caused by loose wiring connections or the boiler running too hot.
Full guide for E50on the Potterton Promax HE Store
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