Genuine Fujitsu 9900599027 Outdoor Thermistor is a temperature sensor in the outdoor unit, reporting the temperature at its position to the control board. If it fails or drifts, the board works from the wrong reading, so the system can heat or cool poorly, cycle oddly or stop and show a fault. The genuine part is the one Fujitsu built this outdoor unit around, so it goes back in as the original did.