BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because a sensor has detected that the internal water or exhaust gases have become dangerously hot.
Why it happens: This fault occurs when the water inside the boiler or the exhaust gases reach a temperature that is no longer safe, triggering an automatic shutdown to prevent damage. The most common reasons are a pump failure that stops water from moving or a blockage in the system, such as a buildup of limescale or sludge, which causes the heat exchanger to trap too much heat.
Check that your radiator valves are open and not turned to zeroCheck that your boiler pressure is between 1.0 and 1.5 bar on the pressure gaugePress and hold the reset button for five seconds
Your boiler has failed to light because it cannot detect a flame, which most likely means there is an issue with the gas supply or a part that ignites the fuel.
Why it happens: This fault occurs when the boiler attempts to start up but fails to detect a steady flame after several attempts. The most common reasons include a lack of gas reaching the burner, a build-up of carbon on the ignition parts, or a blockage in the pipe that carries away waste moisture. In some cases, the sensors inside the boiler may simply be worn out and unable to tell the computer that the flame is lit.
Check that your pre-payment gas meter has credit and is turned on.Verify that other gas appliances in your home, like a hob, are working correctly.Gently press and hold the 'Reset' button for five seconds to see if the boiler restarts.In freezing weather, check if your external plastic condensate pipe has frozen and thaw it with warm water.
Your boiler's computer brain has suffered a serious internal glitch and is unable to communicate with its own sensors or controls.
Why it happens: This fault occurs when the boiler’s main control board experiences a total communication failure between its internal components. It is often caused by an electrical surge, a moisture-related short circuit, or simply the natural degradation of the aging electronics inside the unit. Because the 'brain' can no longer process safety data, the boiler shuts down entirely to prevent any unsafe operation.
Locate the reset button on the front control panelPress and hold the reset button for five secondsRelease the button and wait for the boiler to restart
Your boiler is trying to work but cannot detect enough water moving through the system to operate safely.
Why it happens: This fault occurs when the boiler's internal pump is spinning but is unable to move water through the heat exchanger. The most common reasons are a lack of water pressure in the system, a large pocket of air trapped inside the pump, or a physical failure of the pump motor itself.
Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.If the pressure is too low, use the internal or external filling link to top up the system water.Ensure all radiator valves and isolation valves under the boiler are fully open.Restart the boiler using the reset button once the pressure is corrected.
Your boiler has stopped working because the internal fan isn't spinning fast enough to safely clear away exhaust fumes.
Why it happens: This fault occurs when the boiler's control system senses that the fan isn't rotating at the required speed to safely vent exhaust gases out of the flue. The most common reasons are a build-up of dust and debris inside the fan motor, a mechanical seafing of the bearings, or a communication failure between the fan and the main control board.
Full guide for C1Your boiler's fan is spinning faster than it should, which means the system has stopped running to prevent any damage or unsafe operation.
Why it happens: This fault occurs when the boiler's internal computer detects that the fan is spinning at a higher speed than required for safe combustion. The most common reason is a communication error between the fan and the main control board, or a physical failure of the fan motor's speed regulator. In some cases, a blockage in the flue can also cause the fan to overwork or react erratically.
Full guide for C4on the Worcester Bosch Greenstar 25HE
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 Worcester Bosch 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