BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because it failed to light or stay lit, which has triggered a safety shutdown.
Why it happens: This fault occurs when the boiler tries to light the burner but fails to detect a stable flame after several attempts. The most common reasons are a lack of gas reaching the unit, worn-out spark components that can no longer create a flame, or a sensor that is too dirty to 'see' the flame once it has started.
Check if your gas supply is active and credit is on your meterPress the reset button once to see if the boiler restartsEnsure any external gas valves are in the open position
Your boiler has switched itself off to prevent damage after detecting that it is getting too hot inside the unit.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety switch to shut the system down. The most common reasons are a lack of water pressure, a failing pump that isn't moving heat away from the boiler quickly enough, or trapped air pockets causing the unit to spike in temperature.
Check the system pressure gauge and top up to 1.5 bar if lowEnsure all radiator valves are fully open to allow water to flowBleed any trapped air from your radiatorsPress the reset button after the boiler has cooled down
Your boiler has switched itself off because it has become too hot to operate safely.
Why it happens: This fault occurs when the water inside the heat exchanger reaches a dangerously high temperature, triggering a safety shut-off to prevent damage. The most common reasons include a failing pump that isn't moving water away fast enough, or a buildup of debris and sludge that is restricting the flow through the system.
Check that your radiator valves are open to allow water to flow.Wait for the boiler to cool down completely.Press the reset button once to see if the system restarts.
Your boiler has failed to light the gas or cannot stay lit, meaning your heating and hot water have stopped working.
Why it happens: This fault occurs when the boiler's control board fails to detect a flame after several attempts to fire up. The most common reasons are a lack of gas reaching the burner, dirty internal probes that can no longer 'see' the flame, or a faulty ignition lead that isn't providing the spark needed to start the combustion process.
Check if your gas supply is active by testing another gas appliance like a hob.Ensure your credit is topped up if you have a pre-payment gas meter.Press the reset button on the boiler control panel for 5 seconds.
Your boiler has detected that the water inside is reaching temperatures that are either too high or too low for safe operation, causing the system to shut down.
Why it happens: This fault occurs when the internal sensors detect that the water temperature has strayed outside of its safe operating window, usually due to a lack of circulation or incorrect heat levels. The most common reason is a failing temperature sensor (thermistor) providing inaccurate readings, or a blockage within the heat exchanger that prevents heat from transferring correctly. In some cases, a fault with the gas valve or the main control board can cause the burner to stay on too long, leading to overheating.
Full guide for OTHER FAULTSYour boiler has stopped working for safety reasons and needs a manual reset to try and restart the flame.
Locate the temperature control knob on the front panel.Turn the knob fully anticlockwise to the 'O' (Off/Reset) position.Wait 5 seconds, then turn the knob back to your desired temperature setting.
on the Worcester Bosch Greenstar 15sbi
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