A medical device comprising: a therapy delivery circuit configured to deliver a tachyarrhythmia therapy to a patient's heart via a pacing electrode vector coupleable to the therapy delivery circuit; a sensing circuit configured to: receive a first cardiac electrical signal via a sensing electrode vector coupleable to the sensing circuit; sense a first cardiac event in response to a first crossing of a sensing threshold amplitude by the first cardiac electrical signal; start a blanking period in response to the first sensed cardiac event; and sense a second cardiac event in response to a second crossing of the sensing threshold amplitude by the first cardiac electrical signal; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: start a first quiet timer set to a first time interval in response to the first cardiac electrical signal crossing a noise threshold amplitude; during the blanking period, reset the first quiet timer to the first time interval in response to each crossing of the noise threshold amplitude by the first cardiac electrical signal that occurs prior to the first time interval expiring; determine at least a first parameter of a behavior of the first quiet timer: in response to the sensing circuit sensing the second cardiac event, compare at least the first parameter to electromagnetic interference event criteria; identify the second cardiac event as an EMI event in response to the EMI event criteria being satisfied; update an EMI event count in response to identifying the second cardiac event as an EMI event; compare a value of the updated EMI event count to EMI detection criteria; and withhold the tachyarrhythmia therapy in response to the value of the updated EMI event count satisfying the EMI detection criteria. a therapy delivery circuit configured to deliver a tachyarrhythmia therapy to a patient's heart via a pacing electrode vector coupleable to the therapy delivery circuit; a sensing circuit configured to: receive a first cardiac electrical signal via a sensing electrode vector coupleable to the sensing circuit; sense a first cardiac event in response to a first crossing of a sensing threshold amplitude by the first cardiac electrical signal; start a blanking period in response to the first sensed cardiac event; and sense a second cardiac event in response to a second crossing of the sensing threshold amplitude by the first cardiac electrical signal; and receive a first cardiac electrical signal via a sensing electrode vector coupleable to the sensing circuit; sense a first cardiac event in response to a first crossing of a sensing threshold amplitude by the first cardiac electrical signal; start a blanking period in response to the first sensed cardiac event; and sense a second cardiac event in response to a second crossing of the sensing threshold amplitude by the first cardiac electrical signal; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: start a first quiet timer set to a first time interval in response to the first cardiac electrical signal crossing a noise threshold amplitude; during the blanking period, reset the first quiet timer to the first time interval in start a first quiet timer set to a first time interval in response to the first cardiac electrical signal crossing a noise threshold amplitude; during the blanking period, reset the first quiet timer to the first time interval in response to each crossing of the noise threshold amplitude by the first cardiac electrical signal that occurs prior to the first time interval expiring; determine at least a first parameter of a behavior of the first quiet timer: in response to the sensing circuit sensing the second cardiac event, compare at least the first parameter to electromagnetic interference event criteria; identify the second cardiac event as an EMI event in response to the EMI event criteria being satisfied; update an EMI event count in response to identifying the second cardiac event as an EMI event; compare a value of the updated EMI event count to EMI detection criteria; and withhold the tachyarrhythmia therapy in response to the value of the updated EMI event count satisfying the EMI detection criteria. determine at least a first parameter of a behavior of the first quiet timer: in response to the sensing circuit sensing the second cardiac event, compare at least the first parameter to electromagnetic interference event criteria; identify the second cardiac event as an EMI event in response to the EMI event criteria being satisfied; update an EMI event count in response to identifying the second cardiac event as an EMI event; compare a value of the updated EMI event count to EMI detection criteria; and withhold the tachyarrhythmia therapy in response to the value of the updated EMI event count satisfying the EMI detection criteria.