BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because it has become dangerously hot and needs to cool down to prevent damage.
Why it happens: This fault occurs when the internal temperature of the boiler rises above a safe limit, causing the system to shut down to prevent damage. The most common reasons are a lack of water flow due to a seized pump, a blockage in the heat exchanger, or air trapped within the pipework. Effectively, the heat being produced by the burner is not being carried away into your radiators quickly enough.
Check the pressure gauge on the front of the boiler; if it is below 1 bar, top it up to 1.5 bar using the filling link.Ensure all radiator valves are fully open to allow water to circulate through the system.Press and hold the 'Reset' button for five seconds to see if the error clears once the boiler has cooled down.
Your boiler has failed to light the flame needed to heat your water and home, so it has safely shut itself down.
Why it happens: This fault occurs when the boiler attempts to light several times but the internal sensor cannot confirm that a stable flame has been established. The most common reasons include a lack of gas reaching the unit, worn-out ignition parts that fail to create a spark, or a build-up of carbon on the sensing probe. In many cases, it is simply caused by the burner becoming dirty or a blockage in the system that carries waste gases away.
Check that your gas pre-payment meter (if you have one) has credit.Verify that other gas appliances, like a cooker, are receiving gas.Press and hold the 'Reset' button for five seconds until the display clears.If it is freezing outside, check if the white plastic condensate pipe is frozen and gently thaw it with warm water.
Your boiler is struggling to accurately measure the temperature of the water it is heating, which will likely prevent you from getting hot water through your taps.
Why it happens: This fault occurs when the internal sensor responsible for monitoring hot water temperature starts sending incorrect readings or stops working entirely. The most common reason is that the sensor has suffered a terminal electrical failure or has become heavily corroded after years of constant heat cycling.
Full guide for A7Your boiler's internal control system has lost its connection, meaning it cannot properly talk to its own brain to coordinate heating or hot water.
Why it happens: This fault occurs when the boiler's internal computer can no longer 'talk' to the sensors or the thermostat that tells it when to turn on. The most common reason is a loose or damaged wiring connection inside the unit, or a internal failure on the main circuit board itself. This break in communication means the boiler enters a safety mode because it cannot guarantee it is receiving the correct operating instructions.
Full guide for A8Your boiler is struggling because a small internal identification chip that tells the system how to operate correctly has come loose or is not being recognised.
Why it happens: This fault occurs when the boiler's main control board loses communication with a small memory chip called a code plug, which contains the boiler's essential operating software. The most common reason is a loose connection between the two components, though it can also be caused by a complete electronic failure of the chip or the circuit board itself.
Full guide for B1Your boiler has stopped heating because the internal mechanism that pushes air through the system is not spinning fast enough to allow for safe combustion.
Why it happens: This fault occurs when the boiler's control system detects that the internal fan isn't rotating fast enough to safely expel exhaust gases. It is usually caused by a build-up of dust on the fan blades, a failing motor, or a communication issue from the main control board. If the fan cannot reach the required speed, the boiler will shut down as a safety precaution to prevent fumes from staying inside the unit.
Full guide for C1on the Worcester Bosch ZWB 7-25
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