[NH_Bulldog]

For these units, I believe the basic principal of operation is igniter heats up, gas valve opens and gas is ignited, flame contacts thermocouple which keeps gas valve open and flame going. If thermocouple is bad or not in right location, gas valve thinks there is no flame and shuts to avoid pumping raw gas into unit and possibly leading to explosion as your neighbor walks by with a lit cigarette. I would consider looking at the thermocouple first if unit ignites but doesn’t keep running. Maybe a loose wire, or corroded wire connection? If nothing obvious, have a pro look at it so you don’t take unnecessary risks with safety of the family