An implantable cardioverter defibrillator (ICD) starts a timer set to a time interval in response to a cardiac electrical signal crossing a noise threshold amplitude and resets the timer to the time interval in response to each crossing of the noise threshold amplitude by the cardiac electrical signal that occurs prior to the time interval expiring. A control circuit of the ICD determines a parameter of the behavior of the timer and identifies a sensed cardiac event as an electromagnetic interference (EMI) event based on the parameter. The ICD may detect EMI in response to the EMI event and withhold a tachyarrhythmia detection or therapy in response to EMI detection.