BoilerFaultCodesThe 6 most common faults
Your boiler's computer system has encountered an internal technical glitch that is preventing it from operating correctly.
Why it happens: This fault occurs when the boiler's main control board suffers a critical electronic failure, essentially losing its ability to process data or talk to the sensors. The most common reason is an electrical surge or a component failure within the circuit board's internal memory. Because the 'brain' of the boiler can no longer guarantee safe operation, it shuts the system down entirely.
Press and hold the 'Reset' button for five secondsWait up to 30 seconds for the boiler to restartIf the code persists, check if the fault occurs only when using hot water
Your boiler has detected that there is not enough water circulating inside it, which may prevent your heating and hot water from working safely.
Why it happens: This fault occurs when the boiler's internal sensors detect that the pump is spinning without a sufficient flow of water. The most common reasons are a loss of system pressure, a build-up of air within the unit, or a mechanical failure of the pump motor itself.
Check your boiler pressure gauge; if it is below 1 bar, your system needs to be re-pressurised.Locate your external filling loop (usually two small taps or a braided hose under the boiler).Open the filling taps slowly to allow water into the system until the gauge reaches 1.5 bar, then ensure they are turned fully off.
Your boiler's main control panel has lost communication with the internal computer, meaning it cannot display status messages or operate correctly.
Why it happens: This fault occurs when the boiler's main internal computer loses its connection with the digital screen on the front of the unit. The most common reason is a loose or damaged ribbon cable that transmits data between the two components, or a electronic failure within the display board itself. Essentially, the 'brain' of the boiler is still working, but it can no longer send information to the screen you see.
Press the Reset button for five secondsTurn the main power switch to the boiler off and then back on againCheck if the display panel is firmly clicked into its housing
Your boiler has detected that the flow of air required to safely vent fumes is not moving correctly, and it has shut itself down to prevent unsafe operation.
Why it happens: This fault occurs when the boiler's control board detects that the internal fan is spinning slower than the required safety threshold. The most common reasons are a build-up of dirt within the fan mechanism, a blockage in the air supply tubes, or the motor itself failing after years of use. Because the fan cannot effectively push exhaust gases out of the flue, the boiler shuts down as a vital safety precaution.
Full guide for C1Your boiler is failing to confirm that its internal fan has stopped correctly, which is preventing it from starting up safely.
Why it happens: This fault occurs when the boiler's control system detects that the fan is spinning continuously even when it should be at rest. The most common reason is a mechanical failure within the fan itself or a communication error from the main circuit board that keeps the power running to the motor constantly.
Full guide for C4Your boiler has lost its internal configuration signal, meaning it has forgotten how to control its main heating functions.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, causing the safety switch to trip and shut everything down to prevent damage. The most common reasons are a failed pump that isn't moving hot water away from the heat exchanger quickly enough, or a blockage within the system pipework.
Full guide for d3on the Worcester Bosch 537i
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