Troubleshooting Notes
How Low-Limit Switch, part #12057601, H5887 (140°F-20 low-limit switch) operates:
Switch should be a normally open circuit that closes at 140°F, opens again at fall in temp 140°F-20°F or 120°F. This switch is mounted in the air path of the exhaust and is commonly not malfunctioning but just dirty on the face of the switch. If your wire contacts wiggle a little on the switch, this may actually be okay and is common. A bad switch can be determined by heating the switch face directly with the flame of a match or lighter. After a moment you will hear the switch click. After you hear the click test the switch across the terminals for continuity. If there is no circuit present and you have determined that the switch face is over 140°F, replace the switch. Also, if the switch face is <140°F but still has a circuit, also replace the switch.
This is a F110-90 made from a ceramic material which is high-heat tolerant. Because it's normally open, it's most often referred to as a fan switch. At ambient room temperature this switch is normally open or OFF and needs to detect heat to CLOSE to allow power to flow through the circuit. The switch will close at 20 degrees to restore power and will return to it's normal state at 110 degrees to cut power automatically. Failure of this switch happens when it stops opening or closing at the designated temperature thresholds.
