Your boiler is trying to fine-tune how it burns gas, but it has not been able to complete the check, usually because heat is not moving around the system as well as it should.

DIY fix possible

Look at the pressure gauge on the front of the boiler. The needle should sit between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, follow the filling instructions in your boiler manual or ask an engineer to top it up.Bleed your radiators using a radiator key, starting with the radiator closest to the boiler and working through the house. Air trapped in radiators restricts water flow, and it is a common cause of this fault.Make sure all radiator valves and thermostatic valves are fully open, and check that the boiler's system filter or any isolation valves are not closed. Then switch the boiler off at the mains for 30 seconds, switch it back on, and press the reset button once. If the fault returns, call a Gas Safe registered engineer.