A transistor diagnostic circuit includes a protection transistor output terminal, a fault terminal, and circuitry coupled to the protection transistor output terminal and the fault terminal. The protection transistor output terminal is adapted to be coupled to a current terminal of a protection transistor. The transistor diagnostic circuit is configured to, at start-up, load the protection transistor output terminal to test the protection transistor, and to generate a fault signal at the fault terminal responsive to a voltage on the protection transistor output terminal exceeding a threshold.