configuring the diagnostic circuit to test an on-resistance of a transistor while power is applied to a drain and a source of the transistor from a power source that is external to the diagnostic circuit including configuring the diagnostic circuit to form a first signal that is representative of a drain-to-source voltage of the transistor, to form a second signal that is representative of a drain current of the transistor, and to use the first signal and the second signal to determine if the on-resistance is less than a first value; and

forming the diagnostic circuit and the transistor on a common semiconductor die.