What indicates that a furnace's limit switch may be malfunctioning?

A furnace's limit switch is a critical safety device designed to monitor the temperature within the furnace's heat exchanger. If the limit switch is functioning properly, it will open and turn off the burner if the temperature exceeds a predetermined threshold, preventing overheating and potential damage or hazards.

When the system continuously overheats, it suggests that the limit switch may not be responding appropriately, either due to a mechanical failure or an electrical issue. This failure could prevent the furnace from shutting off the burners when it reaches an unsafe operating temperature, leading to an unsafe situation where the furnace can overheat.

The other options do not indicate a fault with the limit switch. A thermostat set too high isn't necessarily an indication of a malfunction; it merely requests more heat, and as long as the limit switch operates correctly, the system should manage the temperature safely. A blue pilot light typically signifies proper combustion and functioning of the ignition system, while a quiet furnace isn't a definitive indicator of limit switch performance; it could be normal operation if the system is running efficiently.