BoilerFaultCodesThe 6 most common faults
Your boiler is struggling to push water through the system, likely because there is trapped air or a blockage preventing proper flow.
Check that all radiator valves throughout your home are fully open.Bleed your radiators to remove any trapped air, starting from the ones furthest from the boiler.Check your boiler's pressure gauge and top it up to the recommended 1.0–1.5 bar if it has dropped.
Your boiler has detected that the water pressure is too low for it to operate safely, meaning your heating and hot water will likely be limited.
Locate the filling loop (the flexible silver hose) underneath or near the boiler.Open the one or two valve handles slowly until you hear water flowing.Watch the pressure gauge and close the valves once it reaches between 1.0 and 1.5 bar.
Your boiler has stopped working because the water pressure inside your heating system has dropped below the required level.
Locate the filling loop (a flexible metal hose with one or two valves) connected to your boiler or nearby pipework.Slowly open the valve(s) to allow mains water to enter the system until the pressure gauge on the boiler reads between 1.0 and 1.5 bar.Close the valve(s) tightly and restart the boiler.
Your boiler is failing to start because the internal air circulation fan is not spinning fast enough to safely begin the combustion process.
Full guide for F9Your boiler has detected that it is overheating because it cannot effectively move the heat away from its internal components, causing it to shut down for safety.
Full guide for E3Your boiler has detected that a temperature probe monitoring your exhaust gases is not sitting correctly in its intended spot.
Full guide for A3on the Viessmann Vitodens 242-F FB2B
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 Viessmann 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