PRTG Manual say : "When using the auto-discovery function, the Ping Sensor on a device is by default set as the master object for this device. This means that monitoring for the entire device is paused if the Ping sensor is in Down status or in Paused status caused by another dependency.".
OK, Ok... but, when for a device where ping sensor is manually added, how to set it to master ? Or how to make others sensors stop polling when "ping sensor" is down.