BoilerFaultCodesThe 5 most common faults
01
F0 High DIY-safe
Your boiler has detected that the water pressure inside your heating system has dropped too low to operate safely.
What to try first
Locate the external filling loop, which is usually a small silver braided hose with one or two valves located underneath the boiler.Open the valve(s) slowly to allow mains water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close the valve(s) tightly and check that the boiler restarts.
02
F1 High Engineer
Your boiler is trying to start, but it cannot detect a flame, so it has safely shut itself down to prevent gas from building up.
Full guide for F103
F2 High Engineer
Your boiler has lost the ability to monitor the temperature of the water circulating to your radiators, so it has shut down to prevent overheating.
Full guide for F204
F3 High Engineer
Your boiler has detected that water is struggling to flow through its internal heating system, likely due to a build-up of sludge or debris.
Full guide for F305
F4 High Engineer
Your boiler is struggling to accurately measure the temperature of the water being heated for your taps, which is preventing it from providing hot water correctly.
Full guide for F4on the Glow-worm Jaguar Combi
Pressure relief valveExpansion vesselFilling loopIgnition electrodeGas valvePCB
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