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The Goodman 20162906 M4 Primary Limit Switch is a genuine OEM high-temperature safety control for Goodman ACEC961005CNAA furnaces. Mounted on the heat exchanger or burner box, it monitors plenum temperature and shuts off the burners if the furnace overheats — the key protection against a cracked heat exchanger from restricted airflow, a dirty filter, or blower failure.
A limit switch is a calibrated safety device — its trip and reset temperatures must match the furnace design. Generic switches with the wrong setpoint can nuisance-trip (no heat) or fail to trip when needed. This OEM switch carries Goodman's exact temperature rating, restoring proper safety operation.
Moderate (2/5). Disconnect power, photograph and transfer the two wires, then unscrew and swap the switch in its bracket. No gas work required. Verify the furnace cycles and shuts down normally after replacement. When in doubt, consult a licensed HVAC technician.