Heat temperature issues with Goodman GMP 100-4 furnace

The fan switch is set to turn on at approximately 125 degrees and run until the heat exchanger temp drops to around 105 degrees. With age the sensitivity of this switch varies and summer heat can cause the switch to turn on if the outside temp is over 100 degrees. Only the fan is running, not the burner but you can shut off the power to the unit if this is an issue.

have someone check refrigerant charge

Usually you can find the control board on the lower part of the furnace. Turn the power off and remove the
doors. This board is usually located directly in front of the blower motor.

On the control board locate the terminals marked heat/cool/park and Park Lets say the Heat Terminal
has a red wire attached to it and you want a higher
speed.

Red is uasually low speed, remover the red wire from
heat terminal and exchange it with a wire attached to
a terminal lab eled park. Black is usually high spped

Did this problem just start? Or has it been a long term issue? Check for broken ductwork, return air pulling outside air in. Check supply air temps, should be 100-140° or more. And what is the outdoor temp? If it's 0° outside, 74° is doing good. Is the home insulated properly? And there is a small hole behind the t-stat where the control wire comes through the wall. Plug that hole, seal that hole to prevent cold air from fooling the t-stat. The furnace has 3 fan speeds and it may be in low speed. If so it should be in medium speed. I'd suggest a service tech doing this for you unless you are experienced in furnace repair.

Yes. The defrost cycle of a heat pump is actually turning on your airconditioning to heat up the condenser to thaw it out. During this cycle your electric strip heaters "should" be adequate enough to compensate for the a/c being on. At about 30 - 34 degrees outdoor temp there is not enough heat in the air to to adequately heat your home with the heat pump only. Some technitions will use an outdoor thermostat to turn off the heat pump when it is under 34 degrees outside. If you are to cold when its under 34 degrees intall more electric strip heaters.. But if you do this your electric bill may go up significantly.
Welcome to the wonderful world of electric heat pumps (lol).
Good luck.

blowing cold air on this furnace usualy caused by high limit trip, or flame roll out. the first one, (high limit) is from insf. air flow. (blocked filter,bad zone damper?
The second,(flame roll out) caused by bad heat exchanger,blocked vent.ect.

check out the fan switch on the unit and make sure that its the correct one. it should cycloe on @ 120 degrees and cycle off around 100 degrees. Someone may have put in a different one and that is not the approve one.

I am confident the technician(s) have done this but check anyway.
Gas pressure 4-7 inches W.C for natural, 10 - 11 for Propane.
Air filter clean and unrestricted.
Temperature over supply plenum 140 degrees F.
All registers open and unrestricted.
Vent and intake unrestricted and not blocked.
Condensate hoses free of debris and draining.

OK now lets check to see if the furnace is staying on long enough.When it runs for a heat cycle, is the flame staying on until about 2 minutes prior to achieving the set temperature or is it cycling everyfew seconds and never reaching temp? To check you will need to view the flame through a small viewing port of thru the fins in the front door. Watch them for 10 to 15 minutes.

If they are staying on a long time, we will need to see if the room blower is working properly. If they are going out quickly and or cycling. Get the technician on the pone to talk it over. It could be a safety sensor.

If the blower stays on and the burners are on as well, you have a door open somewhere. Just kidding. Remove the thermostst documenting which wires go where. Take the white wire and cross it with the red wire. This will (or should) turn on the furnace. Clamp them together untill the either the house gets warm or you are confident it is not going to reach temperature. If it did get close to the temperature you wanted, it is a thermostat issue. If not, you got me!

And they say we have an easy job! Get beck to us with your results.

I don't know where you are located, but there is a Goodman distributor in Sacramento, CA. Also in Ontario CA. They are a distributor so they may be able to help you or tell where to find someone who will help. Goodman is also known as GMC or Janitrol.