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The Goodman 20162906 M4 primary limit switch is a genuine OEM safety control for the GDS90805D2AA gas furnace. This thermally activated high-limit switch monitors heat-exchanger and plenum temperature, shutting off the burners if the furnace overheats due to restricted airflow, a dirty filter, or blower failure — protecting the heat exchanger and your home.
The genuine Goodman M4 switch is calibrated to the precise trip and reset temperatures your furnace's control board expects. A mismatched generic limit can trip too early (short-cycling, no heat) or too late (dangerous overheating). Because this is a life-safety component, the OEM part is strongly recommended.
Moderate (2/5). Two mounting screws and two spade terminals. Turn off power at the furnace before servicing. A technician can swap it in about 15 minutes; verify the switch temperature rating matches the original.