I have a Carrier gas furnace which developed a problem were the furnace will not shut off. I had swapped out the control board (p/n 302075-302) with an identical spare, it worked, but it last only a month. What could be causing these to fail? My digital thermostat? The gas valve? The fusible link and limit switch are good and I have a spare if needed.

I had added a MOV at the 120AC input circuit because I equated the failure of the first board to a momentary power outage, which is rare here. If I had a schematic of this board I could easily troubleshoot this.

Any suggestions?

Nick D.

Corrected the problem. Spent time drawing a schematic of the board and discovered that the AC to DC converter circuit was not working. Its suppose to produce 12.3VDC, but was only producing 6.7VDC. It's a simple circuit with only one diode, capacitor, resistor and zener diode. It turned out that the 100uF cap was bad, replaced it, all works now.
For others that have the same control board, measure from the "C" terminal to either side of the large resistor, one side should measure about 31VDC and the other side should be 12.2VDC. When mine failed, I was getting 16.3VDC and 6.7VDC respective. The circuit needs 12VDC.

I will be replacing the 100uF cap on my spare board today.

Hope this help you all,
Nick