Compatiblity: This sensor is a stack flue sensor designed for the Pentair Master Temp and Starite Pool Heater. It will also work for the Pentair Mastertemp 125 and all HD heaters within this model. SFS Error: You should consider replacing this part when you see an SFS error indicator in the back of your control board OR if you see an E05 in the front of the Pentair Mastertemp Control board. Location: The stack flue sensor is located within the heater on the black manifold next to the other sensors. The two wires connecting to the sensor will be plugged in when opening. How to Install: Turn off the pump and all the electricity going to the pool heater. Disconnect the two wires of the sensor and unthread the sensor out. Plug your new one in and redo the connections. Purpose: The purpose of this sensor is to measure the temperature of the water within the stack. When measuring the sensor with a multimeter, the sensor should read 3.5 ohms or higher. Swimables Heater Repair Kit for Pentair Mastertemp and Starite Max-E-Therm Pool and Spa Heater - Mega Bundle Includes - Thermistor 42001-0053S, Stack Flue Sensor 42002-0024S, HLS 42001-0063S, AGS 42002-0025S, Igniter 77707-0054, Thermal Regulator 77707-0010, Control Board 42002-0007s, Membrane Pad 42002-0029Z/472610Z, Manifold Oring Set 77707-0120, Bypass Valve 77707-0001, and Air Flow Switch 42001-0061S. Mega Bundle - The perfect Pentair Mastertemp parts list designed to fix most problems within the Pentair Pool Heater. This Pentair Mastertemp 400 parts bundle is designed for the pool professional or the homeowner. Compatibility: This pressure switch is compatible with all BTU's of the Pentair Mastertemp heater parts and Mastertemp 400 parts. It will also work with HD Versions of the heater as well. Affordable: Unlike the OEM, this parts list is the most affordable Master Temp 400 and Sta Rite Repair Kit on the market. Compatible with all Pentair Pool Heater parts. Kit Includes: Thermistor, Stack Flue Sensor, High Limit Switch, AGS, Igniter , Thermal Regulator Kit, Control Board, Membrane Pad, Manifold Oring Set , Bypass Valve, and Air Flow Switch. Best Value - This one purchase can ensure that your Pentair Pool Heaters and Starite Pool Heater will work for years to come. The kit contains enough replacement parts to fix various different issues.