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The Goodman B1370254 Stack Overtemp Switch is a genuine OEM safety limit designed for the GMNT120-5D gas furnace. This switch monitors the flue/stack temperature and shuts down the furnace if combustion gases exceed a safe threshold — a condition that can result from a blocked flue, cracked heat exchanger, or over-firing. As a dedicated overtemp safety, it protects both the equipment and the home from unsafe venting conditions.
The OEM B1370254 is factory-calibrated to Goodman's specified trip temperature and terminal layout for the GMNT120-5D stack location. Generic overtemp switches risk incorrect setpoints that either nuisance-trip or fail to protect the furnace. For a combustion-safety component, OEM is strongly recommended.
Moderate — DIY with caution. Requires locating the switch on the flue/stack, disconnecting terminals, and swapping in the replacement with power and gas off. Because it relates to flue temperature, any repeated tripping should prompt a professional inspection of the heat exchanger and venting before returning the unit to service.