I service a goodmang GMNT 080 furnance and found the fuse blown
Hi, any time you remove the high limit wires the limit switch will show that it is open which starts the indoor blower motor. Your limit was good. It showed it was open on your code chart because the wires were removed. If you are blowing this 3 to 5 amp board fuse, its showing you have a dead short on the low voltage side of the furnace, not the high voltage such as the fan motor would run on. Make sure that the 18gauge thermostat wires coming in to the board are not skinned or have a bare spot touching metal which will cause this. Look it over good where it is pulled through to the board terminals. If it blows as soon as you put a new one in, yes indeed, dead short, 24 volt side. You may have to buy more fuses to find the problem, but any part or safety switch that is 24 volts need to be disconnected until you find the faulty part. Your board could be shorted. Kill the power to the unit. Draw a diagram of the stat wires so you know where they go back. Take them loose. If the fuse holds, the problem will be in the stat wire from there back to the thermostat, and can be the contactor coil shorted on the out door unit or thermostat. After you remove the wires, remove the 4- screws holding the board in. They are ( board ) hard to look at the back side, but work it around to get a look at the circuitry on the back for burned or blacked areas on the board. If there is, the board is the fault. I have seen this many times before. If it is, you may be able to get one at a appliance parts house. Take all model and serial, and board numbers with you. Just keep in mind it is a 24 volt part, control wiring toughing metal, stat, or the board its self. My bet its the control board. Please keep me updated on what you find, and remember to kill power when looking at the back of the board. If I have been of help to you on this, please be kind when rating me as I know you will. I will be here if you need more help.
A/C, Heating, & Refrigeration Contractor