Genuine Fujitsu 9900818005 Outdoor Thermistor Assembly 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. A genuine Fujitsu part is made to the same design as the original, so it goes back in the way the outdoor unit was built.