BoilerFaultCodesThe 6 most common faults
Your boiler has detected that the water pressure inside the system has dropped too low to operate safely.
Locate the filling loop (a silver braided hose) under your boiler or near your cylinder.Open the small valves at either end of the loop to allow mains water into the heating system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves tightly and ensure the filling loop is disconnected if it is a temporary hose.
Your boiler is struggling because it does not have enough water circulating inside it, causing the system to heat up much faster than it safely can.
Check your boiler's pressure gauge and ensure it is between 1 and 1.5 barIf the pressure is low, locate the filling loop (usually a silver braided hose under the boiler)Slowly open the valves on the filling loop to top up the system pressure to 1.5 bar, then close both valves tightly
Your boiler has detected low water pressure and has entered a temporary safety pause to prevent damage while it waits for the system to refill or stabilize.
Check your boiler pressure gauge; it should be between 1.0 and 1.5 bar.If it is below 1.0, locate the external filling loop hose underneath the boiler.Open the valves on the filling loop to allow cold water into the system until the needle reaches 1.5 bar, then securely close both valves.
Your boiler has detected that the water pressure inside the system is low and is waiting for it to reach a safe level before attempting to start again.
Locate the external filling loop, which is a silver braided hose connecting two pipes under the boilerOpen the small valve(s) on the hose until you hear water flowing into the systemMonitor the pressure gauge on the front of the boiler until it reaches between 1.0 and 1.5 barClose the valve(s) tightly and the boiler should automatically clear the code
Your boiler's temperature sensor is failing to provide a reading, which has caused the system to shut down for safety.
Full guide for F.0Your boiler is struggling to accurately measure the temperature of the water returning to it, which prevents it from heating your home safely and has caused it to shut down for protection.
Full guide for F.1on the Vaillant Ecomax Condensing 600/800 series
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 Vaillant 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