My furnace does not blow out any heat but only cold air blows out.. trane XB90 and the diagnostic LED light is flashing 4 times... please help we are cold.. the code for 4 times flahinf light says open limit device.

The main limit control is located on the cell panel just below the burner enclosure, with 2 wires connected. Try tapping with the butt of a screwdriver between the terminals. If this restores operation, look for source of overheating (clogged filter, etc.). Pic of similar control, yours may be somewhat smaller.