NOVELTY - The thermal system (20) has a controller (30) that calculates temperature gradient of heat transfer fluid (22) entering the battery pack (18) using temperature signals (160,162) while the pump (50) is in ON state. A heater diagnostic is executed, only when heating is requested or the absolute value of the calculated temperature gradient does not exceed the calibrated temperature gradient value. The pump is turned off and the cyclic on and off of heater (28) is performed, so as to monitor temperature signals for a calibrated temperature rise when the heater diagnostic is performed.    USE - Thermal system for heating or cooling high voltage direct current (DC) hybrid battery pack used in vehicle (claimed) such as hybrid electric vehicle, extended-range electric vehicle or battery electric vehicle.    ADVANTAGE - The heat generated by the battery pack can be effectively dissipated by circulating a volume of suitable cooling fluid through the battery pack and any associated power electronics, so as to optimize the vehicle performance. Energy-efficient diagnostic method can be provided for diagnosis of the pump without always requiring use of the heater. Thus, unnecessary usage of the heater can be reduced. The thermal system can be used to heat the battery pack, when the battery pack is used in cold weather. Thus, on-board diagnostic (OBD) requirements can be satisfied in a relatively energy efficient manner by minimizing the unnecessary use of electrical energy and excessive cycling of high-voltage components.    DETAILED DESCRIPTION - The controller communicates with temperature sensors (60,62), pump, battery pack, and heater. The controller processes temperature signals from temperature sensors, so as to turn ON the pump in response to a received enabling signal. The passing pump diagnostic code (57) is recorded in controller, if the absolute value of the calculated temperature gradient exceeds calibrated temperature gradient value. An INDEPENDENT CLAIM is included for method for diagnosing thermal system in vehicle.    DESCRIPTION OF DRAWING(S) - The drawings show block diagrams of the vehicle and thermal system.    Battery pack (18)    Thermal system (20)    Heat transfer fluid (22)    Heater (28)    Controller (30)    Pump (50)    Passing pump diagnostic code (57)    Temperature sensors (60,62)    Temperature signals (160,162)