BoilerFaultCodesThe 6 most common faults
Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.
Why it happens: This fault occurs when the water inside the heat exchanger reaches a dangerously high temperature, triggering a safety sensor to cut the power. The most common reasons are a lack of water flow caused by a failed pump, a blockage in the pipework, or air trapped within the system preventing heat from being moved away from the burner.
Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge.Ensure all radiator valves and internal isolation valves are fully open.Bleed your radiators to remove any trapped air pockets from the system.
Your boiler has stopped working because it cannot correctly detect the water pressure, often due to a low water level or a faulty sensor.
Why it happens: This fault occurs when the boiler's internal computer is unable to detect a stable reading from the water pressure sensor, which is essential for safe operation. The most common reason is that the system water level has dropped too low, but it can also be triggered if the sensor itself has become blocked with debris or has failed electronically.
Check the pressure gauge on the front of the boilerIf the pressure is below 1 bar, use the filling loop to top it up to 1.5 barReset the boiler by pressing the 'Reset' button
Your boiler has shut down because it keeps losing water pressure and likely has a leak somewhere in the heating system or the unit itself.
Why it happens: This fault occurs because the boiler has detected a significant and repetitive drop in water pressure, forcing it to shut down to prevent internal damage. It is usually caused by a physical leak within your radiators, pipework, or the boiler's internal components, such as a failing pressure relief valve. If the system cannot maintain a steady pressure four times within a single day, the software triggers this safety lock to alert you to the underlying leak.
Check for visible water leaks under the boiler and around your radiators or valves.Top up the system pressure to 1.5 bar using the external filling loop.Reset the boiler and monitor if the pressure drops again.
Your boiler is struggling to keep its flame lit, which often happens because the drainage pipe is blocked or frozen, or there is an issue with the air supply.
Why it happens: This fault occurs when the boiler's internal computer detects that the flame has unexpectedly gone out during operation. The most common reason is that waste water cannot escape the unit due to a blockage in the drainage pipe, which causes the system to flood and extinguish the burner. It can also be caused by a lack of fresh air reaching the boiler or an unstable gas supply preventing a consistent flame.
Check the white plastic condensate pipe outside for signs of ice or blockages.If frozen, gently pour warm (not boiling) water over the pipe to thaw it.Ensure the external flue terminal is not obstructed by debris or garden foliage.
Your boiler has stopped working because it isn't receiving enough electricity from your home's power supply to run safely.
Why it happens: This fault occurs when the electrical voltage entering the boiler drops below a safe operating level of 180V. The most common reason is a fluctuation or brownout in the local regional power grid, though it can occasionally be caused by a fault within the boiler's internal control board after a power surge.
Check if other appliances in your home are flickering or underperformingEnsure your fuse box / consumer unit has not partially trippedContact your electricity network provider to see if there is a brownout or power fluctuation in your area
Your boiler has detected that the water pressure is too low, which has caused it to stop working to avoid damage.
Why it happens: This fault occurs when the system lacks enough water to circulate safely, triggering a safety shutdown. The most common reasons are a slow leak in the pipework or radiators, or having recently bled your radiators without topping the pressure back up.
Locate the filling loop (usually a silver flexible hose) underneath the boiler.Open the taps on the filling loop slowly until you hear water entering the system.Watch the pressure gauge and close the taps once the needle reaches 1.5 bar.
on the Keston C36
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 Keston 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