This article applies as of PRTG 22

After restarting the PRTG core server service (but not the probe service), many sensors with long scanning intervals show the Unknown status. Why does this happen, and why doesn't PRTG trigger a rescan after the core server restart? 

Reason for Unknown status of sensors after a PRTG core server service restart

Note: This issue only occurs if you restart the PRTG core server service without restarting the PRTG probe service at the same time.

Once the PRTG core server is restarted, it loads and displays the sensor states of the sensors whose last scan was performed less than five minutes ago. Because the PRTG probe service was not restarted, the probes have still been collecting data during the restart of the PRTG core server according to their original schedule.

Depending on a sensor’s scanning interval, it might take several minutes, hours, or days until the probe triggers a new sensor scan and sends the sensor’s data to the PRTG core server.

A sensor with a scanning interval that is longer than five minutes therefore shows the Unknown status until its next scheduled sensor scan.

Troubleshooting solutions

To solve this issue:

• Restart the PRTG probe service as well. The probe then automatically performs a new scan of all sensors when it connects to the PRTG core server.
• Manually scan the affected sensors by using the Scan Now option from the context menu. This triggers an unscheduled sensor scan at the probe.

More

• PRTG Manual: Sensor States
• What to check if sensors are gray?