A fault diagnosing system method for a coolant switching device for a vehicle are provided. The fault diagnosing system includes a first coolant pipe line that passes through an electronic device and an interior heating device. A second coolant pipe line branches off the first coolant pipe line and passes through a radiator. In addition, a coolant switching device is disposed at a branching point at which the second coolant pipe line branches off the first coolant pipe line and an on-board charger charges a vehicle battery and is disposed on the second pipe line. A controller is configured to determine a failure of the coolant switching device and stop the on-board charger from charging the vehicle battery when a temperature of the on-board charger is greater than a first set temperature and a temperature of the electronic device is less than a second set temperature.