BoilerFaultCodesThe 6 most common faults
Your boiler is struggling to circulate water properly because it cannot communicate correctly with its internal water pump.
Why it happens: This code appears when the boiler has been manually put into a specific service setting by an engineer, which changes how the internal pump operates. It is caused by an installer navigating the diagnostic menu and leaving the pump control function active rather than returning it to the standard automatic setting. Because this is a configuration setting rather than a broken component, the boiler is simply waiting for a command to return to normal operation.
Locate the 'Service' or 'Wrench' button on the control panelPress and hold the service button until the display returns to the standard temperature outputTurn the power off and back on at the fused spur switch to reset the electronics
Your boiler has detected that the water inside is getting too hot and has temporarily switched itself off to prevent damage.
Why it happens: This fault occurs when the water inside the boiler gets too hot too quickly, causing the system to shut down for safety. The most common reasons are a lack of water flow caused by a sticking pump or a sensor that has become faulty and is giving a false temperature reading. It can also happen if air is trapped in the heat exchanger or if the system is partially blocked with debris.
Full guide for 2.5Your boiler is struggling to maintain a consistent temperature because the difference between the target heat set on your programmer and the actual water temperature is too large.
Why it happens: This code appears when the boiler has been manually or accidentally switched into a specific service mode that adjustments the burner's firing behavior. It is usually triggered by someone interacting with the internal settings or the control dials in a specific sequence, rather than a mechanical component failing. While in this mode, the boiler pauses its normal operation to allow an engineer to test the temperature gaps between the flow and return pipes.
Turn the temperature control dials fully counter-clockwise and then back to your desired settingPress and hold the 'Reset' button for five secondsSwitch the main power supply to the boiler off and back on again after 30 seconds
Your boiler is struggling to push air through its internal system effectively, which stops it from firing up safely.
Full guide for C1Your boiler's fan is stuck running continuously, which means the system cannot correctly regulate itself and has detected a fault in its cooling or venting process.
Full guide for C4Your boiler has detected that the internal fan responsible for venting exhaust gases is spinning faster than it should, so it has safely shut down to prevent potential damage.
Full guide for C6on the Worcester Bosch ZB 7 27 HE
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