Faulty wire connections.Leaking, kinked inefficient ductwork.Outdoor unit damaged from mans best friend.Obvouis improper installation of equipment.Home fire caused by attic furnace (Mfg. by Consolidated Ind. - recalled furnace).Cracked Furnace heat exchanger.Damage caused from recalled Consolidated attic furnace.Cracked heat exchanger.Cracked heat exchanger.PRheem/Ruud style heat exchanger failure.Plugged air filter. Please note that this style filter typically causes unacceptable overheating of furnace heat exchangers.Plugged indoor coil caused from lack of proper maintenance.Motor failure due to lack of proper preventive maintenance."Split" outdoor coil badly needs cleaning causing extremely high operating costs.Cleaned coil.Furnace installed upsidedown!Installation with no quality control caused this huge air leak. Customer was heating & cooling the outdoors!Improper brazing technique.This chart shows a formicary corrosion test on indoor coils from multiple manufacturers. The ones with the fewest black veins will be the most reliable, even after the warranty ends.Furnace was previouly worked on by other HVAC contractor. Unit was allowing deadly Carbon Monoxide to enter the living space. Safety was bypassed, incompatible components were installed and vent disconnected. Customer choose to replace unit.Furnace safety bypassed by other HVAC contractor.No natural gas eathquake safety pipe installed through side of furnace (code requirement).  Could lead to large gas leak iin the event of an eathquake. This is a very common problem found and easily remidied.Improper installation of condensate drain lines (only requires two drains, not six).