I have a Tempstar Model NTC6100KFG1 gas furnace. Furnace was working fine through the summer blowing the AC. It's been off for a month or two as we've been changing seasons here. When I went to turn on the heat, I found that it would immediately blow the low voltage fuse on the control board.
I replaced the original control board with a Honeywell ST9120U1011 universal control board about two years ago after the original one blew.
Every troubleshooting tip I found said to disconnect the W (heat relay) and R (24V supply) terminals from the thermostat and jumper them on the control board. If the fuse didn't blow, it meant the thermostat wire was shorting somewhere. If it did blow, it meant the control board was bad.
Well, the fuse blew. I'm having a hard time believing that just the heating section of the control board went bad after two year while not being used all summer. Before I go and buy a new one, is there anything else I should check? For instance the igniters or the gas valve? If so, what is the proper procedure for checking these?
Edit: If anyone knows where to find a schematic for this board, that would also be very helpful.
Following the advice of @longneck, I disconnected all the components that make up the heating circuit and reconnected them one by one. The furnace has two igniters as shown below. If I connect only one of the igniters, the control board is fine. But if I connect both of them, the fuse blows. Each igniter is measuring approximately 3Ω. Since they work individually, I don't think there is a short to chassis. Could they be shorting together? What should I check next?