I am attemptinh to assist some friends with there Gas Heater. Model # 376cav036096.. Series J. This heater ignites and comes on only after temperature falls into the 50 degree area... once ignited it only burns for less than 1 minute then goes off again. Thermostat is new and seems to be functioning okay. I have checked filter, gas lines, etc. I have noticed a Red Light on on the right side but am unsure as to what it means. Any assistance would be appreciated

The red light is a status indicator. Observe this when the furnace runs/stops. If it starts flashing, retrieve the 2-digit code as follows: short flashes are 1st digit, long flashes are 2nd digit. Example: 3 short flashes, pause, 4 long flashes, pause/repeating = 34. Look for a label titled "SERVICE" for translation. If the unit runs briefly and stops but light remains steady, the problem is likely in the thermostat.