Communicates the engine coolant temperature to the ECM to ensure proper air/fuel ratio, cooling fan and temperature gauge operation Thermistor placement within the housing matches original design to ensure proper output Proper thermal conductive grease to provide accurate temperature transfer to the thermistor Causes for replacement: check engine light, no/erratic gauge operation, no/erratic cooling fan operation, engine overheating Causes for replacement: rough idle, engine hesitation, poor fuel economy, failed emissions test