BoilerFaultCodesThe 6 most common faults
Your boiler has reached an unsafe temperature and has shut itself down to prevent damage to the internal components.
Why it happens: This fault occurs when water cannot circulate through the boiler quickly enough to carry heat away, causing the internal temperature to spike. The most common reasons are a failed pump that isn't pushing water, a blockage caused by sludge buildup, or air trapped within the system.
Check if any radiator valves have been accidentally closed and open them fullyEnsure your system pressure is between 1 and 1.5 bar on the pressure gaugeGently bleed your radiators to remove any trapped air that might be blocking water flow
Your boiler has detected that a internal sensor responsible for measuring water temperature has stopped working correctly.
Why it happens: This fault occurs when the boiler's internal computer can no longer receive a reliable reading from the sensor that monitors water temperature. The most common reason is that the sensor has physically failed due to age or wear, but it can also be triggered by loose or corroded electrical connections between the sensor and the main control board.
Full guide for A1Your boiler has detected an electrical communication problem with its internal control system which is preventing it from firing up.
Why it happens: This fault occurs when the boiler's internal computer loses the ability to communicate with the sensors that monitor the gas and water flow. It is usually caused by a physical break in the internal wiring or a failure in the electronic components on the main control board. Because the boiler can no longer verify that it is safe to operate, it shuts down immediately as a safety precaution.
Full guide for A5Your boiler has stopped working because it cannot safely clear waste gases through the flue pipe.
Why it happens: This fault occurs when the boiler's internal safety sensors detect that the fan isn't moving enough air to safely vent exhaust gases outside. The most common reasons are a build-up of dirt or debris in the small plastic tubes inside the unit, or the fan motor itself failing after years of heavy use.
Full guide for 669Your boiler has lost communication between its internal computer and a key electrical sensor or motor, meaning it cannot operate safely.
Why it happens: This fault occurs when the boiler's main control board loses its electrical link with a vital internal component, such as a sensor or the fan motor. It is usually caused by a loose connection, a damaged wiring harness, or a failure within the circuit board itself, preventing the boiler from sending or receiving signals. Because the system can no longer monitor safety or performance, it shuts down to prevent damage.
Full guide for ACYour boiler is having trouble communicating with the sensor that monitors the temperature of your hot water cylinder.
Why it happens: This fault occurs when the boiler's main control board loses the electrical connection to the sensor responsible for monitoring your hot water tank’s temperature. The most common reason is that the sensor itself has developed an internal fault or the wiring connecting it to the boiler has become loose or damaged over time.
Full guide for Adon the Worcester Bosch WR325 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