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I have a Carrier Heat Pump model FK4CNF003. I noted that the thermostat wasn't working (no power) so I checked the usual suspects...in bound power etc. before I pulled the top housing off the interior unit. I noted lots of wires on a circuit board in the upper right corner of the unit which seems to route power and signals to/from the thermostat. Looking closely, I noted that the 5 amp fuse was blown so I put in another 5amp fuse and it blew as well. Given the fuse is blown, and I'm assuming the blow fuse is preventing the thermostat from functioning, how should I attempt to backtrack to the source causing the blown fuse and the ultimate casue of the problem? backtracking to find the source a componentcover off the ou cNFmb

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  • vcblack Sep 19, 2008

    Don't see any burn areas on the circuit board:



    Do you agree the blown fuse is the casue of the thermostat not working or could the thermostat be causing the fuse to blow? What does this 5amp fuse protect?...the whole circuit board or is it specific to some component (the blower motor, the thermostat, or other?

  • vcblack Sep 19, 2008

    What does the 5amp fuse protect?

  • vcblack Sep 20, 2008

    There is no noticeable signs of scorched areas on the front or rear of the board...what does the 5amp fuse protect?

  • vcblack Sep 20, 2008



    I'm looking for feedback from someone who is a HVAC professional and knows/understands how to trouble shoot the specific Carrier model in discussion. If you are not qualified in these areas, please don't respond.



  • vcblack Sep 20, 2008

    To: keemo68



    Question; do you have a high degree of confidence that if I just replaced the circuit board the problem would be fixed or could the problem be somewhere off the circuit board?

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You should be able to trace the short to a specific componant on the board itself. the 5amp fuse protects the board itself. trace the circuitry back from the imput side. you should have a multimeter that has a audio alarm that will tell you which circuit is shorted. there is a solid state chip on the imput. start behind it with the test leads. follow the circuit path to each componant. if you cannot find the short that means that the power distribution chip is the fault.

Posted on Sep 20, 2008

  • Ronald Mueller
    Ronald Mueller Sep 20, 2008

    Yes I do I'm a certified a/c tech and i suspect the power distribution chip on the board is the problem the current has to go through it and get's routed to the board. like i said with a multimeter you can check for a short on the board. the meter will have an alarm on the ohm setting and when you test the paths on the board if there's a short you will know it. if you feel confident just replacing the chip then i would just do that it would be cheaper, however you could have power spiked the board and even if you replaced the chip the unit still might not work so the whole board might be a good idea. with out me being hands on it would be hard for me to determine if the whole board is shot....good luck...please rate this solution...thanks and keep me posted

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Hi!!!!

if you wanna know the fuse blown or damege use the tester
or borrow from your friends

check the board if it is burn or damage!!

Posted on Sep 20, 2008

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The source may be the circuit board itself,since the fuse is only 5 amps I would strongly suspect this part, look for any heat damaged area, this is the usual suspect.

Posted on Sep 19, 2008

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  • yadayada
    yadayada Sep 19, 2008

    I really doubt the T-stat is the issue, disconnect it and bypass it just to see if the short goes away.

  • yadayada
    yadayada Sep 19, 2008

    If the circuit board has a short the fuse blows to prevent an electrical fire

  • yadayada
    yadayada Sep 19, 2008

    Also check all the wiring for any burned or damaged insulation at and around the board

  • yadayada
    yadayada Sep 20, 2008

    We provide asssistance based for the most part on a wide range of skills, most of us are skilled in many fields, I am HVAC certified and the answer I gave you is where I would start looking for the problem. Good luck with getting everything to work.

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1 Answer

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Tip

Thermostat wiring terminals and colors


R - The R terminal is the power for the thermostat. This comes from the transformer usually located in the blower section for split systems but you may find the transformer in the condensing unit. For this reason, it is a good idea to kill the power at the condensing unit and the blower section before changing or working on the wiring at the thermostat. If you have a package unit then the transformer is in the package unit.
Red for the R terminal.

RC - The RC terminal is designated for the power for cooling. Some HVAC systems use two transformers. A transformer for cooling and a transformer for heating. In this case the power from the transformer in the blower section would go to the thermostat terminal. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer.
Red for RC terminal.

RH - The RH terminal is designated for the power for heating. See RC above for an explanation. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer.

Y - This is the terminal for cooling or air conditioning and goes to the compressor relay. Typically a thermostat wire pull is made to the air handler on split systems and then this wire is spliced for the separate wire pull which is made to the condenser. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed.
Yellow for Y Terminal.

Y2 - This is the thermostat terminal for cooling second stage if your system is so equipped. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling.
*The most common color I've seen used for this terminal and wire designation is light blue but this varies and is completely up to the installer what color to use. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.

W - This is the thermostat terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler,
White for W Terminal.

W2 - This is the thermostat terminal used for second stage heat. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal. Heat Pumps use staging for auxiliary heat and need a W2 terminal.
*The most common color I've seen used for this terminal and wire designation is brown but this varies and is completely up to the installer what color to use.

G - This is the thermostat terminal used for the fan relay to energize the indoor blower fan. On a split system the blower fan is in the blower section while with a package unit the blower fan is in the outdoor package unit.

Green for G Terminal.

C - This is the thermostat terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. Many digital thermostats require 24 volts for power so the common wire is necessary.
C stands for common and there is no universal color used for this terminal although black is the most common color I've seen.

O or B - These thermostat terminals are for heat pumps and the B thermostat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Most other manufacturers of heat pumps will utilize the reversing valve for cooling and the O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located.

Orange for O and Dark Blue for B depending on the installer of the heat pump and the manufacturer. If you have a Trane, Carrier, Goodman, Lennox, Ducane, Heil, Fedders, Amana, Janitrol, or any other manufacturer other than Rheem or Ruud you will be utilizing the orange wire for reversing valve. Rheem and Ruud will usually utilize the blue wire for reversing valve.

E - This thermostat terminal is for heat pumps and stands for Emergency Heating. If for whatever reason the heat pump condenser fails and it is necessary to run the heat there is an option on heat pump thermostats for emergency heating. Basically this simply utilizes the back-up heat source many heat pumps have to heat the home without sending a signal to the condenser to run for heat.

E - There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the E terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.

X or Aux - This thermostat terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler.

X or Aux - There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.

S1 & S2 or Outdoor 1 and Outdoor 2 - Some thermostats have this terminal and it used for an outdoor temperature sensor. The wire uses for this should be special shielded wire and completely separate form the other thermostat wires.







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1 Answer

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