BoilerFaultCodesThe 6 most common faults
Your boiler has tried to ignite but failed, likely because gas isn't reaching the burner or the flame isn't being detected.
Why it happens: This fault occurs when the boiler attempts to start up but cannot successfully establish or detect a flame. The most common reasons are a lack of gas reaching the unit, a build-up of carbon on the ignition probes, or an internal component like the gas valve failing to open.
Check if other gas appliances in your home, like a hob, are working to confirm gas supply.If you have a pre-payment meter, ensure you have sufficient credit available.Press the 'Reset' button (often indicated by a flame with a cross through it) to see if the boiler restarts.
Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.
Why it happens: This fault occurs when the internal temperature of the boiler exceeds safe limits, triggering a safety sensor to cut the power. It is usually caused by a lack of water flow, often due to a pump failure or a blockage in the main heat exchanger that prevents heat from being carried away from the burner.
Turn the boiler off and allow it to cool for 20 minutesCheck that your radiator valves are open and not turned to zeroPress the reset button (usually marked with a flame or 'reset' symbol) to restart the unit
Your boiler has shut down because the water inside it has become too hot, triggered by a safety sensor to prevent damage.
Why it happens: This fault occurs when the water inside the boiler exceeds a safe temperature limit, causing the safety sensors to shut the system down. It is usually caused by a failure to move heat away from the burner, often due to a broken internal pump or a blockage in the pipework. In some cases, a build-up of limescale or sludge can restrict water flow, leading to rapid overheating.
Check that your radiator valves are open and not turned to zero.Ensure there is enough water pressure by checking the gauge is between 1 and 1.5 bar.Wait 15 minutes for the unit to cool down and try pressing the reset button.
Your boiler has stopped working because the water pressure has dropped too low to operate safely.
Why it happens: This fault occurs when the system loses the water pressure required to circulate heat around your home safely. The most common reason is a slow leak from a radiator valve or joint, though the pressure can also drop if you have recently bled your radiators without topping the water back up.
Locate the filling loop (the flexible silver hose) underneath or near the boiler.Slowly open the one or two small taps on the hose until you hear water flowing.Watch the pressure gauge and close the taps firmly when the needle reaches 1.5 bar.
Your boiler is failing to detect a flame after it tries to start, meaning your heating and hot water will not work.
Why it happens: This fault occurs when the boiler tries to ignite but fails to confirm that a steady flame has been established, causing the system to shut down for safety. The most common reasons are a buildup of carbon on the sensing probes or a fault with the gas valve preventing fuel from reaching the burner. It can also be caused by an unstable gas supply or a worn-out ignition lead failing to carry the spark.
Check that your gas meter is turned on and you have credit if using a pre-payment meterCheck if other gas appliances, like a cooker, are working to confirm gas supplyPress the 'Reset' button (marked with a flame or 'R') on the boiler control panel
Your boiler has stopped working because it cannot safely vent waste gases or confirm that the internal fan is running correctly.
Why it happens: This fault happens when the boiler's internal components can't confirm that harmful waste gases are being safely pushed out of the flue. It usually occurs because the fan has failed to start or the air pressure switch is unable to detect the correct airflow required for ignition. Without this safety confirmation, the boiler will refuse to light to prevent a buildup of dangerous fumes.
Full guide for 02on the Glow-worm Ci Plus
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