The left side of the switch is marked "fan" and the right side "limit",
The switch justs connects the two wires on the left internally and
completes the circuit to the fan. The two wires on the right side are
connected internally until the temperature reaches the high limit safety
setting and the switch opens the circuit, cutting power to the gas
valve. If you look closely at the very bottom center of the switch you
should see where the brass jumper was that I mentioned that needs to be
cut on the new switch. Larry