I have a Goodman 70,000 BTU, up flow gas furnace (GMV950704CXA) with a Honeywell (RTH 7500D) thermostat installed, November 2006. The unit worked fine until outside temperature went into the 20s.

Unit fires and goes through normal sequence and runs for approximately 7 minutes, then cycles down and shuts off as though the house has reached temperature when in reality it has not. The diagnostic LED flashes 3 times indicating the low fire pressure switch is stuck open. In approximately 3 minutes the unit will again repeat this cycle. Thermostat set to 70 or more doesn’t matter, the house does not warm above 67 degrees. The filter was quite dirty (house under construction) cleaned then replaced, with no improvement. Vent and Flue pipe is new 2” X 20’ with 3 elbows and they exit horizontally through the basement wall. The drain trap is clear and functioning. When the outside temperature rises into the 30s this does not occur. There does not seem to be any obstructions in the intake or flue pipe.
I intend to get a little more into this on Saturday, any thoughts would be appreciated.