[onefourthefun]

I have a Thermal Zone furnace with a Nordyne control board, part # 624591-D. The furnace does not operate correctly most of the time. What is confusing is about 5% of the time is does. This is what happening. I apply power to the furnace. I make sure that the thermostat is applying the 24v to the control board to start the heating cycle. The exhaust motor starts. A few seconds later the ignitor starts. A few seconds later I hear the gas relay valve click (may be the control boards sending power to the relay). Then most of the times the cycle stops here. When the cycle does continue gas comes out and lights and then the blower motor starts. When the cycle stops the red LED on the control boards flashes 3 times. The code for this is pressure switch stuck closed. This makes no sense since the furnace manual says this error is detected before the exhaust fan is started. I've also tested the switch and it does open and close correctly. Is the control board going bad? Or the gas relay valve? Or something else?

Jeff