BoilerFaultCodesThe 6 most common faults
01
Overheat Limit Thermostat High DIY-safe
Your boiler has shut itself down completely because the internal water temperature has become too hot.
Why it happens: This fault occurs when water inside the boiler reaches a dangerous temperature, triggering a mechanical safety switch to cut off the gas for your protection. The most common reasons are a failed pump failing to move hot water away from the heat exchanger, or a buildup of limescale and sludge creating 'hot spots' within the unit.
What to try first
Wait for the boiler to cool down for at least 30 minutesLocate the reset button on the underside of the boiler casingPress the button firmly to reset the thermostat and relight the pilot light following the instructions on the boiler door
02
Lockout High Engineer
Your boiler has failed to start after three attempts, often due to a temporary interruption in gas supply or air in the lines, and has turned itself off for safety.
Full guide for Lockout03
Red light flashing High Engineer
Your boiler has failed to start the flame after three attempts and has locked itself down for safety.
Full guide for Red light flashing04
Flashing Red High Engineer
Your boiler has become too hot and has automatically turned itself off as a safety precaution to prevent damage.
Full guide for Flashing Red05
Overheat High Engineer
Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent damage.
Full guide for Overheat06
Overheat Thermostat High Engineer
Your boiler has automatically turned itself off because it became too hot, often caused by a build-up of air in the system or a circulation problem.
Full guide for Overheat Thermostaton the Potterton Prima 30B
Overheat Limit ThermostatPumpFlow ThermostatIgnition leadIgnition electrodePCB (Printed Circuit Board)
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