Following could be the reasons due to which the boiler is not firing -

▪ Flame failure
▪ Carbon deposit on photocell
▪ Electrode clearance too high
▪ Carbon deposit on electrode
▪ Pilot burner not firing
▪ DO tank empty
▪ Water contamination with FO
▪ FO low temperature
▪ Low FO pressure
▪ Incorrect fuel and air ratio
▪ FD fan failure
▪ Filter choked
▪ Defective burner
▪ Nozzle choked