BoilerFaultCodesThe 6 most common faults
Your boiler has lost water pressure, and the internal component used to refill it is likely disconnected or faulty.
Why it happens: This fault occurs when the boiler detects that the system water pressure has dropped too low to operate safely. The specific error indicates that the internal mechanism used to refill the system is either physically disconnected, incorrectly inserted, or the sensor monitoring the water flow into the unit has failed. Essentially, the boiler cannot verify that it is being topped up with water, so it shuts down to prevent damage.
Locate the internal filling link (white key or lever) at the bottom of the boilerEnsure the key is pushed in fully and turned to the locked positionTurn the grey square knob to top up the pressure until the needle is between 1 and 1.5 bar
Your boiler appears to have lost all electrical power, meaning it cannot start up or provide any heating or hot water.
Why it happens: This fault occurs when the boiler's control system is no longer receiving the electrical current it needs to operate. The most common reason is a blown internal glass fuse or a failure within the main circuit board, often triggered by a sudden power surge or water leaking onto electrical components. Effectively, the boiler has lost its 'brain' and cannot communicate with any other parts of the system.
Check if other appliances in your home have powerEnsure the boiler's electrical isolation switch is turned onCheck your home's main fuse box (consumer unit) for any tripped circuit breakers
Your boiler is not reacting when you turn on the heating because it isn't receiving the correct signal to start that specific mode.
Why it happens: This fault occurs when the boiler fails to receive the electrical signal telling it to provide central heating, even though the hot water might still be working. The most common reason is a communication failure from your external room thermostat or a worn-out internal switch that is responsible for toggling the boiler into heating mode.
Check that the room thermostat is turned up higher than the current room temperature.Ensure the mechanical clock or digital programmer is set to 'ON' or 'AUTO'.Replace the batteries in your wireless room thermostat if it has them.
Your boiler has shut down because it has detected its internal temperature is too hot and is now in a safety lockout.
Why it happens: This fault occurs when the water inside the heat exchanger exceeds a safe temperature, triggering a safety sensor to cut the power. The most common reasons are a failed pump that isn't moving water away fast enough or a physical blockage in the system causing the unit to get too hot too quickly.
Check that all radiator valves are fully open to allow water to flow.Wait for the boiler to cool down naturally.Press and hold the reset button for five seconds.
Your boiler has failed to light the flame or detects that the flame has gone out, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler attempts to start up but cannot successfully ignite the gas to create a flame, or if the flame is lost during operation. The most common reasons include a lack of gas supply to the unit, worn-out ignition components that can no longer create a spark, or a build-up of carbon on the sensor that detects the heat.
Check if other gas appliances, like a cooker, are working to ensure gas is reaching the propertyCheck that your prepaid gas meter has sufficient creditPress and hold the reset button for five seconds to see if the boiler restarts
Your boiler has failed to ignite properly, meaning it cannot create the flame needed to provide you with heat or hot water.
Why it happens: This fault occurs when the boiler attempts to start up but cannot establish a stable flame to stay running. The most common reasons include a blockage in the gas supply, worn-out spark electrodes that can no longer ignite the fuel, or a control board that isn't sending the correct signals to the gas valve.
Check that your gas pre-pay meter has credit (if applicable)Check that other gas appliances in your home, like a hob, are working correctlyPress and hold the Reset button for five seconds
on the Worcester Bosch 24/28CDi II RSF
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