BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has reached an unsafe temperature and needs to be cooled down and reset.
Why it happens: This fault occurs when the water inside the heat exchanger becomes too hot, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving around the system or a build-up of sludge that creates a blockage.
Check that your radiator valves are open and not turned to zero.Turn the boiler temperature control knob to the 'O' position.Wait a few minutes for the unit to cool, then turn the knob back to your desired temperature setting to reset the fault.
Your boiler has shut down because it has detected it is getting too hot, usually caused by a blockage or a pump failure.
Why it happens: This fault occurs when internal sensors detect that the water inside the boiler has exceeded a safe temperature limit, triggering an emergency shutdown to prevent damage. The most common reason is that the pump has stopped moving water around your radiators, or a buildup of debris has created a blockage that traps heat inside the unit.
Check that your pump is running and hasn't seizedEnsure all radiator valves are open to allow water to flowWait for the boiler to cool down and press the reset button located underneath the unit
Your boiler is failing to light or stay lit, meaning it cannot heat your home or water.
Why it happens: This fault occurs when the boiler attempts to start up but cannot detect a stable flame after several attempts. The most common reasons are a worn ignition electrode that can no longer spark correctly, or a faulty gas valve that isn't allowing fuel into the burner.
Check if your gas supply is active, for example by testing another gas appliance like a hob.Press the reset button once to see if the boiler restarts.Ensure the condensate pipe (the white plastic pipe outside) hasn't frozen in cold weather.
Your boiler has switched itself off because it has reached an unsafe temperature, usually caused by a circulation issue or a temporary blockage.
Why it happens: This fault occurs when the water inside the boiler exceeds a safe temperature limit, triggering a safety shut-off to protect the internal components. The most common reasons are a failing circulation pump that isn't moving water through the system fast enough or a buildup of debris that has partially blocked the heat exchanger.
Open the control cover on the front of the boilerTurn the temperature control knob fully anticlockwise to the '0' positionWait a few seconds, then turn the knob back to your original desired temperature setting to restart the boiler
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 a dangerously high temperature, triggering a safety sensor to cut the power. The most common reasons are a failed pump that isn't moving the water away fast enough, or a blockage such as air or sludge trapped within the pipework.
Check that all radiator valves are open and not turned to zero.Ensure there are no blockages in the system and that the pump is running.Reset the boiler by turning the temperature control knob to 'Off' and then back to your desired setting.
Your boiler has switched itself off because it got too hot, likely triggered by a safety sensor to prevent damage.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, forcing the system to shut down for safety. It is usually caused by a failure to move heat away from the boiler quickly enough, often due to a broken pump or a blockage in the internal pipework. In some cases, a faulty sensor may mistakenly signal that the unit is overheating even when temperatures are normal.
Open the small control cover on the front of the boiler.Turn the temperature control knob fully anticlockwise to the '0' position.Wait a few seconds, then turn the knob back to your original desired temperature setting to restart the boiler.
on the Glow-worm Micron 40FF
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 Glow-worm 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