BoilerFaultCodesThe 6 most common faults
Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.
Why it happens: This fault occurs when the temperature sensor inside your hot water storage tank detects that the water has dropped to a near-freezing level. To protect the internal components from being damaged by expanding ice, the boiler temporarily pauses operation until the water temperature rises to a safe level. This is typically caused by extremely cold incoming mains water or a storage tank located in an unheated area like a garage or loft.
Full guide for F1Your boiler has lost water pressure and needs to be topped up to work correctly.
Locate the filling loop (a silver flexi-hose) beneath the boilerOpen the valves at both ends of the filling loop to allow water into the systemWatch the pressure gauge and close both valves firmly once the needle reaches 1.5 bar
Your boiler has detected that the internal unit responsible for venting exhaust gases safely is not working, so it has shut down to prevent unsafe operation.
Full guide for F2Your boiler is struggling to clear exhaust gases properly, so it has safely shut itself down to prevent any unsafe buildup.
Full guide for F3Your boiler has switched itself off because it has detected an unsafe rise in internal temperature and needs a professional to check it is venting heat correctly.
Full guide for F4Your boiler has lost the ability for its internal parts to talk to each other, meaning it cannot operate safely and has shut down to protect itself.
Full guide for F13on the Glow-worm Ultracom sxi
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 Glow-worm 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.
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