BoilerFaultCodesThe 6 most common faults
Your boiler is struggling to push hot water through your radiators because the external pump that circulates the water is not working correctly.
Why it happens: This fault occurs when the boiler sends power to the external pump but detects that water isn't moving through the system as expected. The most common reason is that the pump has seized due to a build-up of debris or internal mechanical failure, preventing it from spinning. In some cases, it can also be triggered by a communication error between the boiler's control board and the pump's electrical wiring.
Full guide for D,11Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Locate the external filling loop, which is a silver braided hose with one or two valves connecting your boiler to the mains water supply.Open the valve(s) to allow water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close the valves tightly and check that the boiler restarts.
Your boiler has lost the ability to read the temperature of the water inside it, so it has shut down to prevent overheating.
Full guide for F.00Your boiler has lost the signal from one of its internal temperature sensors, causing it to stop operating as a protective measure.
Full guide for F.01Your boiler has stopped working because a sensor that measures water temperature is providing an incorrect signal, so the system has safely shut down to prevent overheating.
Full guide for F.10Your boiler has stopped working because it is unable to accurately read the temperature of the water returning to it from your radiators.
Full guide for F.11on the Vaillant ecoTEC plus Regular
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