BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.
Why it happens: This fault occurs when the water inside the boiler exceeds a safe temperature, causing the system to shut down for safety. The most common reasons are a pump failure preventing water from moving through the system or a blockage caused by a build-up of sludge and debris. It can also be triggered by a faulty sensor that is incorrectly reporting the water temperature to the control board.
Check that your radiator valves are fully openEnsure your boiler pressure is between 1 and 1.5 bar on the gaugeReset the boiler by turning the control knob to 'RE' or pressing the reset button
Your boiler has tried to light several times but cannot establish a flame, meaning it has shut down for safety to prevent gas build-up.
Why it happens: This fault occurs when the boiler's control system attempts to start a flame but fails to detect one after several tries. It is often caused by a lack of gas reaching the burner, a dirty or damaged ignition spark, or a sensor that can no longer 'see' the flame even if it has lit. In some cases, a blocked flue or internal component failure prevents the correct mix of air and gas required for ignition.
Check if your gas supply is active by testing another gas appliance like a hob.If you have a prepay meter, ensure you have sufficient credit.In freezing temperatures, check if the external condensate pipe has frozen.Reset the boiler by turning the mode knob to 'RE' and back to your desired setting.
Your boiler has stopped working because the electrical power coming into your home is at a lower voltage than it needs to operate safely.
Why it happens: This fault occurs when the electrical supply entering the boiler drops below 190 volts, which is too low for the internal components to function. While it is sometimes caused by a failing control board within the boiler, it most commonly originates from an issue with the National Grid supply or the electrical wiring leading to your home.
Check if other appliances in your home are acting strangely or lights are flickeringContact your electricity network provider to report a potential power supply issueOnce the external power issue is resolved, reset the boiler using the 'RE' dial or button
Your boiler has detected that the water pressure inside the system is too low to run safely and has switched itself off.
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 reasons are a slow leak somewhere in your central heating pipework or radiators, or a loss of air pressure within the expansion vessel which is meant to regulate the system. In some cases, it can also happen if you have recently bled your radiators without topping the water level back up using the filling loop.
Locate the filling loop (usually blue-handled levers) underneath the boiler.Slowly open the valves until you hear water entering the system.Watch the pressure gauge and close the valves when the needle reaches 1.5 bar.Restart the boiler if it does not reset automatically.
Your boiler has detected an issue with the sensor that measures the temperature of the water leaving the unit, preventing it from heating your home safely.
Why it happens: This fault occurs when the internal sensor responsible for monitoring the heat of the water leaving the boiler fails to send a clear signal back to the control board. The most common reason is that the sensor has physically degraded or developed an electrical short, meaning the boiler can no longer safely tell if the water is getting too hot. In some cases, loose wiring or a connection issue between the sensor and the main computer can also trigger this shutdown.
Full guide for F4Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators.
Why it happens: This fault occurs when the boiler's control board stops receiving a reliable signal from the sensor that tracks the temperature of the water coming back from your heating system. The most common reason is that the sensor has physically failed or developed a loose connection, preventing the boiler from safely monitoring heat levels.
Full guide for F5on the Ideal Logic Max Combi
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
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.