An implantable medical device capable of sensing cardiac signals and delivering cardiac electrical stimulation therapies is enabled to detect a short circuit condition on a sensing or therapy vector. A cardiac signal is sensed by a sensing module coupled to electrodes. A control module detects a short circuit condition in response to a significant drop in amplitude of far-field cardiac events coincident to near-field cardiac events. In some instances, the short circuit condition is detected in response an abnormal impedance on the far-field sensing vector and/or a matching morphology of the far-field cardiac electrical signal including the coincident FF cardiac event in addition to the significant drop in amplitude of the coincident FF cardiac events.