BoilerFaultCodesThe 6 most common faults
Your boiler has detected that waste water cannot drain away properly and is backing up inside the main heating unit.
Check the plastic drain pipe leading away from the boiler for any obvious blockages.If the pipe runs outside, check if it has frozen during cold weather and thaw it with warm (not boiling) water.Ensure the external termination of the pipe is not submerged in water or debris.
Your boiler has received an external signal to stop running, likely from a smart thermostat, heat pump interface, or safety switch that is temporarily preventing it from firing up.
Check if your thermostat or heating controller is set too low or has run out of batteries.Ensure any external switches or 'Frost Stat' units haven't been triggered by mistake.Restart the boiler using the power button to see if the external signal clears.
Your boiler has switched itself off because the water pressure in the system has dropped too low to operate safely.
Locate the internal or external filling loop (usually two braided silver hoses or blue handles under the boiler).Slowly open the valves until you hear water flowing and watch the pressure gauge.Close both valves tightly once the pressure gauge reaches between 1.0 and 1.5 bar.
Your boiler is trying to light, but it cannot detect a flame and has shut down as a safety precaution.
Check that your gas meter has credit and the isolation valve is openCheck if other gas appliances in your home (like a hob) are workingPress the Reset button on the boiler control panel once
Your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.
Locate the external filling loop, which is usually a flexible silver braided hose with one or two valves located underneath the boiler.Open the valves slowly to allow mains water to enter the system until the pressure gauge on the boiler front displays between 1.0 and 1.5 bar.Close the valves tightly and check for any visible leaks around the joints.
Your boiler has stopped working because it cannot correctly monitor the temperature of the water leaving the unit due to a sensor or wiring fault.
Full guide for F.5on the Viessmann Vitodens 200-W B2KF Combi
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
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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