BoilerFaultCodesThe 6 most common faults
Your boiler has tried to light several times but cannot detect a flame, causing the system to shut down for safety.
Why it happens: This fault occurs because the boiler's control system cannot confirm that a flame has successfully lit after several attempts. It is usually caused by a lack of gas reaching the burner, a worn-out ignition spark electrode, or a dirty sensor that can no longer 'see' the flame even if it is there. The boiler shuts down as a safety precaution to prevents unburnt gas from building up inside the unit.
Check that your gas emergency control valve is in the 'on' positionIf you have a pre-payment meter, ensure you have credit availableCheck if other gas appliances in your home, such as a hob, are working correctlyPress the reset button once to see if the boiler restarts
Your boiler has no power and is completely unresponsive, likely due to an electrical supply issue or a blown fuse.
Why it happens: This fault occurs when electrical power fails to reach the boiler's internal controls, leaving the unit completely lifeless. The most common reason is a blown fuse in the external wall switch or a surge that has tripped the boiler's internal protection, often caused by a failing component like the pump or fan. In some cases, the interference filter has failed, preventing electricity from flowing through the circuit safely.
Check if your home's main electrical consumer unit has a tripped circuit breakerEnsure the boiler's external isolation switch (fused spur) is turned onReplace the 3-amp fuse in the external isolation switch with a new one
Your boiler has stopped working because the water level inside the system has dropped too low and needs topping up.
Why it happens: This fault occurs when the internal water pressure drops below the minimum level required for the boiler to operate safely. The most common reason is a slow leak from a radiator valve or a pipe joint, but it can also happen if you have recently bled your radiators and didn't top up the system afterwards. In some cases, a component inside the boiler like the pressure relief valve may be letting water out of the system gradually.
Locate the filling loop (usually a silver flexible hose) underneath the boiler.Slowly open the one or two small tap handles until you hear water running.Watch the pressure gauge and close the taps firmly once it reaches between 1 and 1.5 bar.
Your boiler has detected a communication problem or a fault with the sensor that controls your central heating temperature.
Why it happens: This fault occurs when the boiler's main computer loses touch with the sensor that monitors your heating water temperature. It usually happens because the sensor has failed internally or the electrical wires connecting it to the control board have become loose or damaged. Without this vital temperature data, the boiler shuts down as a safety precaution to prevent overheating.
Full guide for CH light on PCB 40 flashingYour boiler has detected a fault with the temperature sensor that controls your hot water, meaning it cannot accurately measure how hot the water is.
Why it happens: This fault occurs when the boiler's internal computer receives an incorrect electrical signal from the hot water temperature sensor. It usually means the sensor has developed a short circuit or the wiring connecting it to the main control board has become damaged or loose. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.
Full guide for HW light on PCB 40 flashingYour boiler has shut down because it detected that the water inside has become too hot, causing a safety switch to trigger.
Why it happens: This fault occurs when the water within the heat exchanger reaches a dangerously high temperature, triggering a safety sensor to cut off the gas for your protection. The most common reasons for this are a lack of water circulation caused by a failing pump, a blockage in the pipework, or a sensor that has become faulty and is giving a false reading.
Full guide for Overheat 'stat trippedon the Ideal Response
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 Ideal 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