BoilerFaultCodesThe 6 most common faults
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 lost power because an internal fuse has blown, likely caused by an electrical fault in the pump.
Why it happens: This fault occurs when an electrical component within the boiler draws too much current, causing a safety fuse on the control board to blow and cut all power. The most common reason is that the central heating pump has developed an internal short circuit or has seized, often due to a build-up of debris or water leaking into its electrical housing.
Full guide for Mains fuse F2 on PCB 40 blownYour boiler has stopped working because the internal fan, which safely helps the system breathe, is not spinning.
Why it happens: This fault occurs when the boiler's control system attempts to start the fan but detects that it isn't spinning or reaching the required speed. The most common reasons are a mechanical failure of the fan motor itself or a communication issue where the main circuit board fails to send power to the fan.
Full guide for 82Your boiler is receiving power but cannot ignite because it isn't detecting the correct gas pressure required to start the burner.
Why it happens: This fault occurs when the boiler's internal computer tries to start the heating process but detects that the gas isn't reaching the burner at the correct strength. It is usually triggered by a mechanical failure in the gas valve or a communication error where the control board fails to send the right signals. In some cases, a faulty fan or an external blockage can also prevent the system from reaching the necessary pressure to ignite safely.
Full guide for 83on 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