Trouble Shooting Boiler Flame Goes Off Quickly

If Your Boiler Flame Goes Out Soon After Starting, It could possibly be due to a flue blockage to determine that Check the flue for blockage then Check dilution fan operation.
Otherwise it could also be a failure in the low-pressure hot water circuit. In that case check operation of the high-temperature cut-out.

If your boiler has a fault in flue gas system It might be that during the start sequence, the flue gas fan does not run.
In that case check the control circuit.
It could also be that the fan has stopped during the operation.
Check the fan, make any necessary fixes, and restart.