Genuine Fujitsu 9900817015 Outdoor Heat Exchanger Thermistor (Red) is the thermistor that tells the outdoor unit control board the temperature at the point it is fitted. A failed sensor leaves the board working blind, which shows up as poor heating or cooling, odd cycling or a fault code on the unit. The genuine part is the one Fujitsu built this outdoor unit around, so it goes back in as the original did.