TM 9-4940-342-14&P Table 5-1.  Troubleshooting - Continued Malfunction Probable Causes Corrective Action 2.  Low fire switch closed, burner does not ignite - Continued n.  Electrode cracked. n.  Remove  burner  and  cycle  low  fire start  switch.  Observe  spark,  if  spark is   weak   remove   and   inspect   elec- trodes. Replace cracked electrodes. o.  Air in fuel line. o.  Burner    cycles,    oil    pressure    gage fluctuates   (see   malfunction   2,   step f.    Allow  burner  to  recycle  until  oil displaces air and ignites. p.  Time delay contact block mechanically bound. p.  Observe  block  to  see  if  cocked  on guides.  Free  block  to  work  freely  on the guides. 2A.  High fire switch closed, burner does not ignite. a.  Fuel valve or lines restricted or plugged. a.  See malfunction 2, step e. b.  Fuel valve solenoid coil loose or burned out. b.  See malfunction 2, step f. c.  Burner nozzle is plugged. c.  Remove  burner  and  inspect  nozzles. Remove   nozzle   and   clean   orifice, nozzle strainer and supply pipe. d.  Flame safeguard contacts dirty. d.  See malfunction 2, step h. e.  Wire loose or broken. e.  Check  conductors  of  control  circuits 5  and  6  from  control  terminal  board to  high  fire  switch  for  tightness  and continuity.     Check     conductor     of circuit   5   from   terminal   board   to flame   safeguard   for   tightness   and continuity. Tighten, repair or replace conductors as necessary. f.   Air in fuel lines, occurring when unit is restarted after fuel lines disassembled. f.   With  high  fire  switch  turned  on,  oil supply   pressure   will   drop   causing low  fire  burner  to  trip  out.  Restart low   and   high   fire   until   all   air   is displaced by oil. g.  Defective high fire start switch. g.  See malfunction 1, step h. 3.  Air pressure switch contacts open, inadequate air. (Switch closes at 0.15 a.  Blower inlet obstructed. a.  Visually inspect blower inlet. Remove obstruction. WC) b.  Air sensing static tube plugged. b.  Remove  ¼-inch  copper  tubing  from air  flow  switch  and  connect  water tube  manometer  to  tubing.  Start  fan, if  manometer  does  not  read  at  least 0.15-inch  the  switch  will  not  close. Clear   static   tubing   by   blowing   air through tube from switch side. c.  Static tube not aligned. c.  Same    as    malfunction    3,    step    b. Remove burner and align opening of tube to face toward the blower end of air  duct.  Assure  tube  is  centered  in duct.   Rest   air   sensing   is   toward blower and slightly downward. d.  Switch setting incorrect. d.  Switch   will   not   shut   down   burner when obstruction is placed in front of blower   housing.   Increase   set   point by    turning    adjustment    clockwise. Check   the   trip   point   by   covering blower inlet. When setting switch by this method, hold cover over inlet for only  a  few  seconds  or  until  black smoke  emits  from  exhaust.  If  switch is  set  properly  burner  will  trip  in- stantly.     Switch     will     not     allow burner   to   ignite   because   setting   is 28