Step1: Disconnect power and gas supply using lockout/tagout procedure, and let equipment cool completely for safety.
Step2: Remove access panel and locate existing hi-limit thermostat assembly inside unit housing.
Step3: Disconnect wiring, noting terminal positions and labeling connections for reinstallation.
Step4: Replace with new unit, secure mounting hardware, and reconnect wires to matching terminals.
Step5: Restore panel, restore power, test operation, and ensure hi-limit cutoff operates properly before full use.