The boiler has noticed the water pressure in your central heating system has drifted outside its normal range, which is usually a simple top-up issue but can stop the heating working if left.

DIY fix possible

Check the pressure gauge on the front of the boiler with the system cold. On most Vitodens models it should read roughly 1 to 1.5 bar, and the boiler's manual gives the exact range.If the reading is below the range, top it up using the filling loop as described in the Viessmann user manual. Close the valves afterwards and make sure the loop is fully disconnected.Watch the gauge over the next day or two. If the pressure keeps dropping, or you see damp patches, drips or water around radiators or the boiler, stop and call a Gas Safe registered engineer to find the leak.If the pressure rises above the range, do not open any valves to release water. Turn the boiler off and call a Gas Safe engineer, as this can point to an expansion vessel or pressure relief valve fault.