BoilerFaultCodesThe 6 most common faults
Your boiler is overheating because the water inside is heating up too quickly, causing the system to shut down temporarily for safety.
Why it happens: This fault occurs when the water inside the boiler heats up at an abnormally fast rate, suggesting that the heat isn't being moved away from the burner quickly enough. The most common reasons are a failing pump that cannot circulate the water or a physical blockage, such as limescale or sludge, trapped within the main heat exchanger.
Check that your boiler pressure gauge is between 1 and 1.5 barEnsure all radiator valves are fully open particularly the lockshield valvesInspect the condensate pipe for signs of freezing if the weather is very cold
Your boiler is struggling to move water because there is air trapped in the system or the water pressure has dropped too low.
Why it happens: This fault occurs when the boiler detects that there is no water passing through the internal pump, causing it to spin without resistance. The most common reasons are a significant drop in system pressure or a large pocket of air becoming trapped inside the pump itself, which prevents it from being able to circulate heat. To protect the internal components from overheating or burning out, the boiler triggers a safety shutdown.
Check the pressure gauge and top up the system to 1.5 bar using the filling linkGently bleed air from your radiators using a radiator keyPress the reset button on the boiler control panel
Your boiler has lost its connection with the thermostat, meaning they are no longer speaking to each other to control your heating.
Why it happens: This fault occurs when the wireless link between your boiler and your wall thermostat is broken, preventing the boiler from receiving instructions to turn on. The most common reasons are dead batteries in the thermostat unit or an obstruction like a new piece of metal furniture blocking the radio signal between the devices.
Check if your thermostat batteries need replacingEnsure the thermostat is within range and has no obstructionsTurn the power to the boiler off and back on to restart the connection
Your boiler has stopped working because it cannot detect a flame, meaning there is likely a disruption to the gas supply or an issue with the ignition process.
Why it happens: This fault occurs when the boiler tries to light but fails to detect a sustained flame, causing it to shut down for safety. The most common reasons include a lack of gas reaching the burner, dirty electrodes that can no longer sense the heat, or a blockage in the drainage pipe that prevents the boiler from firing correctly.
Check that your gas meter is in credit and the gas emergency control valve is open.Check if other gas appliances, like a cooker, are working to confirm gas supply.Press the 'Reset' button on the boiler control panel for five seconds to restart the ignition sequence.During freezing weather, check that the white plastic condensate pipe outside is not frozen or blocked.
Your boiler's internal computer has encountered a critical electronic glitch that is preventing it from running safely.
Why it happens: This fault occurs when the boiler's central control board suffers a critical electronic failure, meaning it can no longer communicate with the internal components. The most common reason is a hardware malfunction within the circuitry or an electrical surge that has damaged the delicate microprocessors. In some cases, loose or damaged wiring connections between the board and the sensors can also trigger this shutdown.
Locate the 'Reset' button on the front control panelPress and hold the button for 5 seconds, then releaseWait up to 10 minutes to see if the boiler restarts and the error clears
Your boiler’s internal computer has lost contact with a small identification chip that tells it how to operate correctly.
Why it happens: This fault occurs when the boiler's main control board can no longer read the small identification chip, known as a code plug, which contains the unit's specific settings. The most common reasons are a loose connection caused by natural vibrations over time or a physical failure of the chip or the circuit board itself.
Full guide for b1on the Worcester Bosch Greenstar CDi Compact
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
Avoid the next call-out bill
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