BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because there is not enough water pressure in the central heating system.
Why it happens: This fault occurs when the water level inside your heating system drops below the minimum level required for the boiler to operate safely. The most common reasons are a slow leak from a radiator valve or pipework, or a failure in internal components like the expansion vessel which helps manage system pressure.
Locate the silver filling loop (flexible hose) underneath or near the boiler.Slowly open the one or two taps on the loop until you hear water flowing.Watch the pressure gauge until it reaches between 1 and 1.5 bar, then close the taps tightly.
Your boiler has entered a safety lockout mode and has stopped running to protect its internal components.
Why it happens: This fault occurs when the boiler fails to detect a flame after several attempts, causing it to shut down for safety. The most common reasons include a worn-out ignition spark that can no longer light the gas, or a faulty gas valve that isn't letting fuel into the burner. In some cases, the internal control board may be failing to send the correct electrical signals to start the sequence.
Locate the boiler's electrical power switch (fused spur) on the wall nearby.Switch the power off for approximately 10 seconds.Switch the power back on and wait to see if the red light clears and the boiler fires up.
Your boiler has stopped working for safety reasons and needs to be reset to try and fire up again.
Why it happens: This fault happens when the boiler attempts to start up but fails to detect a steady flame, causing it to shut down for safety. It is usually caused by a lack of fuel reaching the burner or a component failing to generate the spark needed to ignite the gas. In some cases, the boiler may actually be lighting, but a faulty sensor is failing to tell the internal computer that the flame is present.
Locate the reset switch on the control panelPress and release the reset switchWait at least 15 seconds for the boiler to attempt to restart
Your boiler has shut itself down for safety because it either got too hot or failed to light the gas properly.
Why it happens: This fault occurs when the boiler's internal safety sensors detect that the unit has become dangerously hot or that the gas has failed to ignite after several attempts. The most common reasons are a lack of water circulation causing the heat exchanger to trap too much heat, or worn-out ignition parts that can no longer create a spark to light the burner.
Locate the reset button (often marked with a flame symbol or 'R') on the control panelPress and hold the reset button for several seconds and wait for the boiler to restartCheck that your gas pre-pay meter has credit and that other gas appliances like your hob are working
Your boiler has encountered an internal error and needs a simple restart to try and clear the fault.
Why it happens: This fault occurs when the boiler's internal electronics detect a communication error or a safety interruption that prevents the system from starting. The most common reason is a temporary glitch in the control board or a sensor that has tripped due to minor overheating or a failed ignition attempt. It acts as a safety barrier to ensure the boiler does not operate until a qualified person or a manual reset has cleared the issue.
Locate the electrical spur switch or plug for the boilerTurn the power off for approximately 30 secondsTurn the power back on to see if the flashing light stops and the boiler restarts
Your boiler's internal thermometer is broken, meaning it cannot accurately tell how hot the water is and has stopped working as a safety precaution.
Why it happens: This fault occurs when the internal sensor responsible for monitoring water temperature fails due to electrical degradation or internal corrosion. Over time, the sensitive components inside the thermistor can wear out or become coated in limescale, preventing the sensor from sending the correct electrical signal to the boiler's main control board.
Full guide for Faulty CH thermistoron the Glow-worm Swift Flow 100
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