Your boiler has received a warning signal from an external device connected to its extension module, which usually means something outside the boiler needs checking, though the boiler may still be working normally.

DIY fix possible

Turn off the boiler at its power switch or fused spur and wait 30 seconds.Check any external device wired to the boiler's extension module, such as a room thermostat, alarm contact, or external control, for loose wires or obvious damage, without opening the boiler casing.Turn the power back on and press the reset button on the boiler or controller. If the code returns, stop and call a Gas Safe registered engineer.