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 has sensed that the water inside is getting too hot too quickly and has shut down as a safety precaution.
Why it happens: This fault occurs when the water inside your boiler cannot circulate fast enough to carry heat away from the burner, causing the internal temperature to spike instantly. The most common reasons are a failing pump that isn't pushing the water round, or a blockage caused by sludge and debris restricting the flow within the system.
Check that your radiator valves are fully openEnsure the boiler pressure is between 1 and 1.5 bar on the gaugeCheck if the pump is making a humming sound, which might suggest it is stuck
Your boiler has failed to ignite properly, meaning it cannot create a flame to provide heating or hot water.
Why it happens: This fault occurs when the boiler attempts to light several times but fails to detect a stable flame. The most common reasons include a lack of gas reaching the burner, contaminated ignition electrodes that can no longer create a spark, or a faulty gas valve preventing the fuel from flowing correctly.
Check if other gas appliances, like a hob, are working to ensure gas supply is active.Inspect the condensate pipe outside for signs of freezing if the weather is very cold.Press the 'Reset' button on the boiler control panel for five seconds.
Your boiler is having trouble communicating with the internal clock or plug-in timer, preventing it from knowing when to turn on.
Why it happens: This fault happens when the boiler's main control board loses touch with the plug-in timer or external programmer responsible for scheduling your heating. It is usually caused by a loose connection in the internal wiring or a mechanical failure within the clock component itself, meaning the boiler doesn't receive the 'on' signal it expects. While the boiler's hardware might be fine, it stays idle because it can no longer see your heating schedule.
Check that the plug-in timer or programmer is firmly pushed into the boiler fasciaReset the boiler by holding the reset button for five secondsEnsure any external room thermostats have fresh batteries and are calling for heat
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
on 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