BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because a sensor has detected that the internal water or exhaust temperature has become dangerously high.
Why it happens: This fault occurs when the water inside the boiler gets too hot or the exhaust gases exceed safe limits, triggering a safety shutdown to prevent damage. The most common reasons are a lack of water circulation caused by a failed pump, a blockage in the heat exchanger, or a buildup of air or sludge in the system. Essentially, the heat being generated isn't being moved away quickly enough, causing the unit to overheat.
Check that your radiator valves are open and not turned off.Check that the boiler pressure is between 1 and 1.5 bar on the pressure gauge.Restart the boiler by holding the 'Reset' button for five seconds to see if the fault clears.
Your boiler is failing to detect a flame and has safely shut down to prevent unburned gas from flowing.
Why it happens: This fault occurs when the boiler's control system cannot confirm that a flame has successfully lit, leading it to shut down for safety. The most common reasons include a lack of gas reaching the burner, worn-out ignition components that fail to spark, or a dirty internal sensor that can no longer 'see' the flame.
Check if you have credit on your gas meter or if other gas appliances in the house are working.Inspect the white plastic condensate pipe outside for signs of freezing and thaw it with warm water if necessary.Press the 'Reset' button on the boiler fascia for five seconds to see if the system restarts.
Your boiler has shut down because the water in the heating system has become too hot, usually due to a lack of circulation or a blockage.
Why it happens: This fault occurs when the water leaving the boiler exceeds its safe temperature limit, causing the system to shut down for safety. The most common reasons are a pump failure that prevents water from moving, or a physical blockage within the pipework or heat exchanger that traps heat inside the unit.
Check that all radiator valves (TRVs) are turned to the highest settingEnsure the system pressure is between 1 and 1.5 bar on the gaugeBleed all radiators to remove trapped air that could be blocking water flow
Your boiler's computer may have encountered a temporary glitch or a more serious internal component failure.
Why it happens: This fault occurs when the boiler's main control board experiences a communication error or an internal electronic failure, preventing it from processing safety checks. It is often caused by a sudden power surge, moisture ingress, or simply the age of the electrical components leading to a system crash. Think of it like a computer freezing; the brain of the boiler can no longer send or receive the signals needed to operate safely.
Locate the reset button on the front control panel.Press and hold the reset button for 3 to 5 seconds.Wait for the boiler to restart and see if the code clears.
Your boiler has shut itself down for safety because it cannot successfully light or maintain a flame.
Why it happens: This fault occurs when the boiler's internal computer can no longer detect a stable flame during the ignition process. The most common reasons are worn-out sparks that fail to light the gas, a dirty sensor that can't 'see' the flame, or an inconsistent gas supply preventing a steady fire from forming.
Check if your other gas appliances (like a cooker) are working to ensure gas supplyEnsure your decorative gas fire or meter valve hasn't been accidentally turned offPress and hold the reset button for five seconds once gas supply is confirmed
Your boiler has stopped working because the internal fan is not spinning fast enough to safely clear away exhaust fumes.
Why it happens: This fault occurs when the boiler's control system detects that the fan is not spinning at the correct speed to safely expel waste gases through the flue. The most common reasons include a buildup of dust and debris within the fan motor, a loose electrical connection, or a failing control board that is unable to send the correct power signal to the fan.
Full guide for C1on the Worcester Bosch Greenstar 25 HE Combi
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