Methods and systems consistent with this invention detect a failed short thyristor in a solid-state controller for delivering power to a load during reduced-voltage operation. Methods and systems consistent with this invention detect a voltage across the thyristor, and indicate a failed short thyristor when the absolute value of the voltage across the thyristor remains below a threshold value during a predetermined period of time. Methods and systems consistent with this invention detect a failed open thyristor in a solid-state controller during full-voltage operation. Methods and systems consistent with this invention detect a voltage across the thyristor, and indicate a failed open thyristor when the absolute value of the voltage across the thyristor exceeds a threshold value.