Auxiliary power unit is blowing cooled air when my truck is off the heat is not working but if i turn my truck on the heating works great its just when i turn off my truck APU is working but not blowing heat but everything else works just not the heat
Check Fuel Level,but that shall be OK.
Check for Fuse on Night Heater.
Check if Fuel comes to Night Heater (Maybe Pump stopped due to Electrical or Mechanical Fault)
There musst be some kind of Spark Plug. Its not sparking but just to give ya an Idea. Check it.