Goodman 80 000 BTU 80% AFUE LoNox Downflow Gas Furnace GDH80804BX Control board stores the last five diagnostic codes in memory; simple push-button activation out - puts the fault history to a flashing red LED Low constant fan allows homeowner to activate the low heat speed to efficiently circulate air throughout the home. Self-adjusting feature automatically adjusts furnace to high- or low-stage operation based on outside tem - perature without an outdoor temperature sensor Certain models available with low NOx emissions Dual-diameter tubular heat exchanger