BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it is getting too hot inside, usually to prevent internal damage.
Why it happens: This fault occurs when the internal temperature of the boiler rises above a safe limit, causing a safety sensor to cut the power. The most common reason is a blockage or a failing pump that prevents water from flowing through the system quickly enough to carry heat away from the burner. In some cases, it can also be triggered by a build-up of air or debris within the heat exchanger.
Check if your radiator valves are open to allow water to flow.Check the boiler pressure needle is in the green zone (between 1 and 1.5 bar).Press the 'Reset' button on the boiler controls to see if the fault clears after cooling down.
Your boiler is failing to light the gas, meaning it has shut down to prevent unburnt gas from building up.
Why it happens: This fault occurs when the boiler's internal computer doesn't see a flame after several attempts to light the gas. The most common reasons include a lack of gas reaching the unit, worn-out spark electrodes that can no longer create a flame, or a dirty sensor that fails to 'see' the heat even when it is present.
Check if you have credit on your gas meter or if other gas appliances are workingIf it is freezing outside, check if the white plastic condensate pipe is frozen and thaw it with warm waterPress the Reset button for five seconds until the display clears
Your boiler has experienced a sudden technical error and has locked itself to prevent damage, requiring a manual reset to attempt to clear the fault.
Why it happens: This fault occurs when the boiler's internal computer detects a critical failure during the ignition process or a sudden loss of flame during operation. The most common reasons are worn-out ignition components failing to spark or the gas valve not delivering the correct amount of fuel to the burner.
Press and hold the 'Reset' button for five secondsWait for the blue light to stop flashing and stay solidCheck if your gas supply is active (e.g. check a gas hob or prepay meter credit)
Your boiler has reached an unsafe temperature and has automatically shut down to prevent damage and ensure your safety.
Why it happens: This fault occurs when water inside the boiler cannot flow away quickly enough, causing it to rapidly overheat and trigger a safety shutdown. The most common reasons are a seized internal pump that isn't pushing water around the system, or a blockage such as trapped air or a build-up of sludge. In some cases, a faulty sensor may simply be misreading the temperature even if the water itself is fine.
Check if any radiator valves have been turned off and open them fullyEnsure your system pressure is between 1 and 1.5 bar on the pressure gaugeOnce the boiler has cooled down, press the reset button
Your boiler has detected an issue with how it clears exhaust fumes, causing it to stop running for safety.
Why it happens: This fault occurs when the boiler's safety sensors detect that exhaust fumes are not being cleared safely through the outdoor flue pipe. It is typically caused by a blockage in the air pressure tubes, a build-up of debris in the fan, or a mechanical failure in the switch that monitors airflow.
Full guide for E1Your boiler's temperature sensor has failed or has a wiring issue, which means the system cannot safely monitor how hot the water is or provide heating and hot water.
Why it happens: This fault occurs when the internal sensor responsible for monitoring the temperature of the water leaving the boiler fails to send a clear signal to the main control board. The most common reason is that the sensor's internal electrical resistance has broken down over time, or the wiring connecting it has become loose or damaged. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.
Full guide for E2on the Worcester Bosch Greenstar 25Si
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