BoilerFaultCodesThe 6 most common faults
01
F1 High DIY-safe
Your boiler has switched itself off because it has reached an unsafe temperature and needs to cool down before it can be restarted.
What to try first
Check that all radiator valves are fully openEnsure the boiler pressure is between 1 and 1.5 bar on the gaugePress the reset button once the boiler has cooled down for 20 minutes
02
F4 High DIY-safe
Your boiler has stopped working because it failed to light a flame or the drainage pipe for waste water is blocked.
What to try first
Check if your gas supply is active (e.g., check other gas appliances or your meter credit).Inspect the external white plastic condensate pipe for signs of freezing or blockages.Press the 'Reset' button on the boiler control panel once to see if the fault clears.
03
F3 High Engineer
Your boiler's internal thermometer has stopped working correctly, meaning the system can no longer monitor the water temperature and has shut down as a precaution.
Full guide for F304
F5 High Engineer
Your boiler has shut down because it can no longer safely measure the temperature of the exhaust gases leaving the system.
Full guide for F505
F8 High Engineer
Your boiler is struggling to push air through its system correctly, so it has safely shut itself down to prevent any damage.
Full guide for F806
F9 High Engineer
Your boiler has detected that the internal air extraction fan is spinning when it should be switched off, which prevents the system from starting safely.
Full guide for F9on the Heatline Vizo Plus
Overheat thermostatCirculation pumpThermal fuseIgnition ElectrodeGas ValvePrinted Circuit Board (PCB)
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 Heatline 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