A pacemaker comprising: a pulse generator configured to generate pacing pulses; a cardiac electrical signal sensing circuit configured to sense R-waves attendant to depolarizations of a ventricle of a patient heart; a sensor configured to produce an intraventricular signal comprising atrial event signals; and a control circuit in communication with the sensor, the cardiac electrical signal sensing circuit and the pulse generator and configured to: set a timer to a conduction check time interval; operate in an atrial synchronous ventricular pacing mode by: sensing an atrial event from the intraventricular signal; and setting an atrial-ventricular pacing interval in response to each sensed atrial event; and controlling the pulse generator to generate a pacing pulse upon expiration of the atrial-ventricular pacing interval; after expiration of the conduction check time interval, switch to an asynchronous ventricular pacing mode by: setting a ventricular pacing interval to a base pacing rate interval; and controlling the pulse generator to generate a pacing pulse upon expiration of the ventricular pacing interval; determine that a threshold number of ventricular pacing pulses are delivered during the asynchronous ventricular pacing mode; determine that atrioventricular block detection criteria are satisfied in response to the threshold number of ventricular pacing pulses being delivered; and responsive to the atrioventricular block detection criteria being satisfied, switch back to operating in the atrial synchronous ventricular pacing mode. a pulse generator configured to generate pacing pulses; a cardiac electrical signal sensing circuit configured to sense R-waves attendant to depolarizations of a ventricle of a patient heart; a sensor configured to produce an intraventricular signal comprising atrial event signals; and a control circuit in communication with the sensor, the cardiac electrical signal sensing circuit and the pulse generator and configured to: set a timer to a conduction check time interval; operate in an atrial synchronous ventricular pacing mode by: sensing an atrial event from the intraventricular signal; and setting an atrial-ventricular pacing interval in response to each sensed atrial event; and controlling the pulse generator to generate a pacing pulse upon expiration of the atrial-ventricular pacing interval; after expiration of the conduction check time interval, switch to an asynchronous ventricular pacing mode by: setting a ventricular pacing interval to a base pacing rate interval; and controlling the pulse generator to generate a pacing pulse upon expiration of the ventricular pacing interval; determine that a threshold number of ventricular pacing pulses are delivered during the asynchronous ventricular pacing mode; determine that atrioventricular block detection criteria are satisfied in response to the threshold number of ventricular pacing pulses being delivered; and responsive to the atrioventricular block detection criteria being satisfied, switch back to operating in the atrial synchronous ventricular pacing mode. set a timer to a conduction check time interval; operate in an atrial synchronous ventricular pacing mode by: sensing an atrial event from the intraventricular signal; and setting an atrial-ventricular pacing interval in response to each sensed atrial event; and controlling the pulse generator to generate a pacing pulse upon expiration of the atrial-ventricular pacing interval; sensing an atrial event from the intraventricular signal; and setting an atrial-ventricular pacing interval in response to each sensed atrial event; and controlling the pulse generator to generate a pacing pulse upon expiration of the atrial-ventricular pacing interval; after expiration of the conduction check time interval, switch to an asynchronous ventricular pacing mode by: setting a ventricular pacing interval to a base pacing rate interval; and controlling the pulse generator to generate a pacing pulse upon expiration of the ventricular pacing interval; setting a ventricular pacing interval to a base pacing rate interval; and controlling the pulse generator to generate a pacing pulse upon expiration of the ventricular pacing interval; determine that a threshold number of ventricular pacing pulses are delivered during the asynchronous ventricular pacing mode; determine that atrioventricular block detection criteria are satisfied in response to the threshold number of ventricular pacing pulses being delivered; and responsive to the atrioventricular block detection criteria being satisfied, switch back to operating in the atrial synchronous ventricular pacing mode.