A voltage of a first pin that is one of several pins of an external connector of a system is measured, while the first pin is un-driven except for being pulled to ground through a first resistance, and a second pin of the external connector is being used as a power supply rail of the system. The measured voltage is compared to a short circuit threshold and in response to that threshold being exceeded, the power supply voltage on the second pin is reduced. In such an embodiment, no test stimulus needs to be applied to any of the pins of the external connector. Other embodiments are also described and claimed.