I assume it does not try to light? Without a wiring diagram I can offer few suggestions. Most fan drafted tanks do not have a pilot, rather, electronic ignition. It surely must have some sort of fan "prove" switch so that it will not light without proper vent pressure/ suction from the fan. The pilot would not stay lit if the thermocouple was bad or circuit opened
This is what I was talking about. The pressure switch proves the fan is running. To me it sounds like the fan is constantly running unless he unplugs it. So it's trying to light the main burner but will not until the switch closes.
Causes are debris in orifices at the pressure switch but most likely at the fan itself. The hose could be cracked or a birds nest in the vent itself (happens all the time!)
