The boiler is struggling to finish checking its flame sensor after a self-test, so it may pause or refuse to fire until the check completes successfully, and this is usually temporary.

DIY fix possible

Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar if it is low, using the filling loop as shown in the manual.Switch the boiler off at its power supply or fused spur, wait 30 seconds, then switch it back on to restart the calibration.Make sure the gas supply tap is fully open and that no other gas appliance is causing a sudden pressure drop, then wait 10 minutes and see if the boiler runs normally.If the code returns, stop and call a Gas Safe registered engineer, as the flame sensor may need cleaning or replacing.