A diagnostic apparatus for a coolant control valve including a housing having a plurality of ports, a valve member provided to be rotatable in the housing, and a driving device rotating the valve member between two end positions, the diagnostic apparatus including at least one position sensor provided on an external surface of the housing for sensing a position of the valve member, a determination device for determining whether or not the valve member is stuck using the position of the valve member sensed by the position sensor and a controller for controlling the driving device.