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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a gas pac, model number pgd 42c0902d, serial number

Customer Question

I have a gas pac, model number pgd 42c0902d, serial number(###) ###-#### With air conditioning on and fan select switch in automatic, the house fan continues to run after compressor has cycled off. What component is faulty?
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.
Hello. The fan relay contacts are burnt and stuck closed, or the printed circuit board that the fan relay is mounted on is bad and keeping the ran relay coil energized. That is the most likely situation, 90% or better chance of that.The other 10% could be caused by several different things. If you have Red, Yellow, White, and Green terminals on the thermostat and where they connect inside the furnace or air handler, you can remove the G wire, and if the fan stops, the problem is with the thermostat. IF the fan does not stop then the problem is in the printed circuit board or its attached relay most likely. The remaining potential is that the high temperature limit switch that trips when the furnace over heats is open. that will cause the fan to run all the time. I would need a well focused, close up photo of the wiring diagram glued to the inside of the units service panel to coach you how to diagnose that.... you would need a $30 type multi meter for that.------------ We are on the honor system here. Please remember to rate my service before you leave today. It is how I earn a living, and does not increase your charges. If you need anything else, just let me know. You can continue asking follow-up questions within your agreed on limit at no additional cost.