Your boiler has lost its connection to your room thermostat or controls, so it may not be responding to heating requests correctly, but this is usually a fixable wiring or thermostat issue.

DIY fix possible

Check that your room thermostat has working batteries, or that its display is lit if it is mains or wired powered.Look at the thermostat wiring at both ends and make sure no wires are loose or disconnected. Switch the boiler off at the isolator or fused spur before touching any terminals.Confirm the thermostat is an OpenTherm-compatible model and that it is set to OpenTherm mode, then switch the boiler back on and reset it using the reset button or by power cycling it.If the code returns, leave the boiler off and contact a Gas Safe registered engineer.