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Do you have several fuses s we can try a few things until we find the short?
You can get some at a Walmart or auto parts for testing.Can be 5 amp or so if they don't have 3 amps.
Lets start by disconnecting the complete ac circuit at furnace. Remove R wire and The Y wire from board. If there is no Y terminal on board. disconnect the yellow wire where it connects from thermostat in the furnace.
ith those wires disconnected, turn power on to furnace and push door button in. If fuse pops that means we have a problem with board.If not we will move on.
Excellent.Next lets go to outdoor unit. Find where the control wires first enter the unit and remove wire nuts and disconnect the Y and C .This will take the outdoor unit out of the circuit. Turn on power and call for ac. If fuse blows we know we have a short in wire or something between unit and furnace.If not we will go into the condensing unit.
Go ahead and reconnect everything at indoor unit. Just disconnect the yellow and red at outdoor. I will wait for you. Go ahead and give me model on outdoor unit so I can pull a manual and be looking at the internal circuit of outdoor unit in case fuse does not blow with the yellow and red disconnected.
Will have manual on desk.Standing by
I have to leave for the night but will check in tomorrow morning.
OK looks like there are only 3 or 4 components between where wires enter unit and the contactor.
1.Low pressure switch
2.High Pressure switch
3.Hot gas sensor
Manuals shows some are optional so tell me which you actually have.Follow that yellow wire all the way to contactor.
Also while following the yellow wire, look for any place that the yellow wire could have gotten against a hot gas copper line and melted the sheathing allowing wire inside to touch the copper pipe.
See diagram below: