Inlet Blocked

• Always check the easiest option first, even if it seldom turns out to be the cause of the problem. If air cannot be drawn into the system, the blower has no air to move: Check to make sure no obstruction has fallen against the inlet to the system, such as a flat panel of siding dropped from the external wall. Plants can grow to block inlets, especially bushy shrubs, and trailing plants such as ivy can block the vent or even grow inside the duct work. If you find any obstructions, remove them. If the inlet is unobstructed, check to make sure the filter is clean and clear; if it is clogged, replace it.
  
  

Blower Malfunctioning

• Once you've ascertained that fresh air can reach the blower, turn on the system and make sure that the fan inside the blower unit is in fact rotating. If it isn't, first check that the fuse is good; if it is blown, it probably indicates there is another problem downstream that must be addressed. If the fuse is good, visually inspect the wiring circuit; it is possible that animals have gnawed on the wiring to the blower to the point that it can't transmit electricity. If the wiring appears good, check to see if the impellers move freely. Turn off the system, and then spin the fan with your finger; if the impellers are trapped, discover the cause and rectify the problem. Electronic components fail more speedily when they are located in adverse environments, such as under a trailer house, so it is possible that the blower unit needs to be replaced entirely. Unless you are a very competent home handyperson, diagnosing wiring problems and changing blower units are jobs for licensed, bonded HVAC engineers.
  
  

Duct Work Blocked

• If treated air is blowing out of some vents but not others, this suggests a blockage in the duct work serving the vents that do not emit cooled air. Use graph paper to make a scale drawing of the trailer house, showing the inside walls, and then draw in all the vents. Mark those that do work --- and those that don't --- in different colors. Follow the duct work beneath the trailer and draw a "map" of how it is routed, joining the vents to the blower with an accurate plan of the ducting. It should be obvious --- from matching the vents that do not blow treated air with the plan --- where the blockage is; it will be at the place in the system nearest the blower that serves all the dead vents and none of the live ones. Open up that part of the system and remove the obstruction, remaining aware at all times that any animals that have taken up residence in your ducts will probably bite.
  
  

Duct Work Porous

• Although most sheet metal duct work is made of galvanized material, corrosion can still attack it in unfavorable environments. Shifting in the trailer house can cause joins between sections of ducting to separate. Animals can attack an amazing variety of materials --- even those that are obviously inedible, such as duct work --- and it is possible that rats or other pests have gnawed holes in the ducting system. Flexible extents of duct work used to connect the system to the blower, and to join together interconnect parts of the system, sometimes come off if the clamps that originally held them in place have corroded and fallen off. In all these cases, the pressure created by the blower is lost before the treated air reaches the vents. In this situation, you are literally paying to cool the outside air in the void under your trailer house. Visually inspect the duct work system, looking for leaks, and repair or replace any components as necessary.