Genuine Daikin 1823578 Outdoor Four-Way Valve Coil is the electrical coil that drives the outdoor reversing valve, switching the system between heating and cooling on command from the control board. When it fails, the unit may be stuck in one mode, struggle to change over, or lose output. A genuine coil drives the valve the way your Daikin expects, holding reliable mode changes — where a non-genuine part can drive it weakly or unreliably.