BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has detected that it is getting far too hot inside.
Why it happens: This fault occurs when the water temperature inside your boiler exceeds a safe limit, causing the safety sensor to shut the system down to prevent damage. The most common reasons are a failed internal pump that isn't moving water away quickly enough or a buildup of sludge and limescale that is restricting flow through the heat exchanger.
Check that the boiler pressure is between 1 and 1.5 bar on the needle gauge.Ensure all radiator valves are fully open to allow water to flow.Press the 'Reset' button on the front of the boiler once the system has cooled down.
Your boiler is failing to light the gas or isn't detecting that the flame has started, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler's internal computer tries to start the burner but cannot confirm that a flame is safely lit. The most common reasons are a lack of gas reaching the unit, worn-out sparking components that fail to ignite the fuel, or a dirty sensor that cannot see the flame even if it is present.
Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.If it is freezing outside, check that your external condensate pipe hasn't frozen and blocked.Press and hold the reset button for five seconds to see if the boiler restarts.
Your boiler's internal computer has encountered a communication error or software glitch that is preventing it from running safely.
Why it happens: This fault occurs when the boiler's main control centre suffers a sudden communication breakdown between its internal electronic components. It is typically caused by a component failure on the circuit board or a loose connection in the wiring harness that prevents the computer from verifying its own safety checks. Because the 'brain' of the boiler can no longer guarantee safe operation, it shuts the system down immediately.
Press and hold the reset button for five secondsWait for the boiler to complete its restart sequenceIf the code persists, turn the main power switch to the boiler off and back on again
Your boiler has detected that there is not enough water circulating through the system to operate safely.
Why it happens: This fault occurs when the boiler's internal sensors detect air pockets or a significant drop in water pressure, preventing the pump from moving water around. The most common cause is a leak in the system or radiators that have recently been bled, leaving the boiler with insufficient water to operate safely. Without enough water to act as a lubricant and coolant, the pump 'runs dry' and the system shuts down to prevent heat damage.
Check your boiler's pressure gauge to see if it is below 1 barLocate the filling loop (usually silver braided hoses under the boiler)Slowly open the valves until the pressure reaches 1.5 bar, then close them tightly
Your boiler is struggling to accurately measure the temperature of your hot water, which usually means your taps will only run cold or slightly warm.
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 main control board. The most common reason is natural wear and tear or corrosion on the sensor's probe, which prevents it from reading how hot the water is, causing the boiler to shut down the burner for safety. In some cases, it can also be triggered by a loose or damaged connection in the wiring that links the sensor to the rest of the unit.
Full guide for A7Your boiler's internal computer has lost its connection with the main control board, preventing the system from operating.
Why it happens: This fault occurs when the boiler's internal computer modules cannot talk to each other, similar to a loose connection between a computer and its monitor. It is usually caused by moisture affecting the electrical contacts, a vibrating wiring harness that has worked loose, or a physical failure of the central control board's circuitry.
Full guide for A8on the Worcester Bosch Greenstar 30HE
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