BoilerFaultCodesThe 6 most common faults
Your boiler has got too hot and has automatically shut down for safety to prevent damage.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety sensor to cut the power. It is usually caused by a pump failure that stops water from moving or a blockage in the system that prevents heat from being carried away fast enough. In some cases, air trapped in the heat exchanger can also cause a localized hotspot that trips the sensor.
Check that your radiator valves are open to allow water to circulate.Ensure your system pressure is correct (usually between 1 and 1.5 bar).Press the reset button on the front of the boiler once it has cooled down.
Your boiler has failed to light, which most likely means the gas supply is interrupted or there is a blockage preventing the burner from starting.
Why it happens: This fault occurs when the boiler attempts to ignite several times but fails to detect a flame, causing the system to shut down for safety. The most common reasons include a lack of gas reaching the burner, worn-out spark electrodes, or a buildup of carbon on the sensor that 'sees' the flame.
Check if your gas meter has credit or if other gas appliances in the house are working.If it is freezing outside, check if the white plastic condensate pipe leading outdoors is frozen and thaw it with warm water.Press the Reset button on the boiler for five seconds to see if it restarts.
Your boiler's internal computer has encountered a glitch or a hardware failure, preventing it from running properly.
Why it happens: This fault occurs when the boiler's main control board loses its ability to process data correctly or experiences a communication breakdown between internal components. It is often caused by a sudden electrical surge, moisture entering the control box, or simply the natural degradation of the solder points and microprocessors over time.
Switch the boiler off at the fused spur or power switchWait 30 seconds and switch the power back onPress and hold the Reset button for 5 secondsIf the code persists, contact a Gas Safe engineer to inspect the internal electronics
Your boiler is having trouble measuring the temperature of your hot water, which usually means it cannot heat it safely or effectively.
Why it happens: This fault occurs when the internal sensor responsible for monitoring the temperature of your hot water fails to send a reliable signal to the boiler's control board. The most common reason is that the sensor has physically degraded over time or developed an electrical short, meaning the boiler can no longer verify if the water is at a safe temperature. In some cases, it may also be caused by damaged or corroded wiring connecting the sensor to the main computer.
Full guide for A7Your boiler's internal computer has lost its connection with the main control board, meaning the system cannot communicate correctly to heat your home.
Why it happens: This fault occurs when the boiler's central brain loses its ability to talk to the other internal electronic components. It is usually caused by a loose connection in the internal wiring or a component failure on the main circuit board that prevents data from flowing through the system. Think of it like a loose HDMI cable on a TV; the power is on, but the picture cannot get through.
Full guide for A8Your boiler's internal computer memory chip is loose or missing, meaning it cannot identify which model it is supposed to be.
Why it happens: This fault occurs when the small memory chip, known as a code plug, becomes loose or fails to communicate with the boiler's main control board. Without this chip, the boiler essentially forgets its own identity and safety settings, leading to an immediate shutdown for security reasons. The most common cause is either physical vibration loosening the connector or an electrical failure within the chip itself.
Full guide for b1on the Worcester Bosch Greenstar Ri
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
Monthly cover means no unexpected bills when your boiler breaks down.
Affiliate links. We may earn a commission at no extra cost to you.