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The Goodman 20162906 M4 Primary Limit Switch is a genuine OEM high-limit control for AMH80604BXAA gas furnaces. Mounted on the heat-exchanger bulkhead, this auto-reset thermal switch monitors supply-air temperature and cuts the gas valve if the furnace overheats — usually from restricted airflow, a dirty filter, or blower failure. It automatically closes again once temperatures return to normal, protecting the heat exchanger from thermal stress.
The primary limit is calibrated to a specific open/close temperature for your furnace's cabinet. The OEM switch guarantees the correct trip point; a mismatched generic can nuisance-trip or, worse, allow overheating. As a safety-critical part, we recommend genuine OEM.
Difficulty: Easy to Moderate. Power off, disconnect the two wire terminals, remove the two mounting screws, and install the replacement in the same orientation — about 15–25 minutes. Clear any airflow restriction that caused the original trip before restarting.