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The Goodman 20162906 M4 primary limit switch is a genuine OEM overheat safety control for the Goodman GKS91155DXAA gas furnace. Mounted on the heat exchanger/plenum, this normally-closed thermal switch opens the gas valve circuit if supply-air temperature climbs too high — protecting the furnace and home from overheating caused by dirty filters, blocked ducts, or blower failure. A failed limit switch causes short cycling, no heat, or nuisance lockouts.
Limit switches are calibrated to a specific trip temperature matched to the furnace's heat-exchanger design. Genuine OEM units carry the correct cut-out point and mounting geometry; generic switches may trip early, late, or not at all — a serious safety concern. For a safety control, OEM is strongly recommended.
Power down the furnace, remove the two mounting screws, transfer the spade connectors, and reinstall. A DIY-capable owner can manage it, but confirm correct part fit and never bypass a limit switch.