Sporlan 700611 SA 217 Sight Glass 30067439 890 this L300F high limit switch this L300F high limit switch is engineered to monitor heat exchanger temperatures and interrupt the burner circuit if conditions exceed a safe thermal threshold of 300°F The York S1 031 01252 000 Temp Sensor is a high precision OEM replacement part designed for critical thermal monitoring in residential and light commercial HVAC systems high sensitivity indicator element that remains dark green when the system is dry and shifts to bright yellow if moisture is detected install the SA 217 in the liquid line immediately after the filter drier and before the expansion valve