What is this?

This knowledgebase contains questions and answers about PRTG Network Monitor and network monitoring in general.

Learn more

PRTG Network Monitor

Intuitive to Use. Easy to manage.
More than 500,000 users rely on Paessler PRTG every day. Find out how you can reduce cost, increase QoS and ease planning, as well.

Free Download

Top Tags

View all Tags

My sensors show the Unknown state after a PRTG core server service restart. What can I do?



After restarting the PRTG core server service, but not the PRTG probe service, a lot of my sensors show the Unknown state. These sensors all have long scanning intervals. What is the reason for this behavior and why does PRTG not trigger a rescan of these sensors after the PRTG core server restart? What can I do?

prtg restart restart-service sensor-state unknown

Created on Jan 2, 2020 8:08:54 AM by  Maike Guba [Paessler Support] (2,404) 2 1

Last change on Aug 21, 2023 10:05:37 AM by  Yasodhara Das [Paessler Support]

3 Replies

Accepted Answer



This article applies as of PRTG 22

Reason for Unknown state 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 state 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.


Created on Jan 2, 2020 8:15:04 AM by  Maike Guba [Paessler Support] (2,404) 2 1

Last change on Aug 21, 2023 10:06:22 AM by  Yasodhara Das [Paessler Support]



I have the same problem, but when I restart the probe, the status changes to green and after een half hour back to grey???All the sensors! I am since 2 days now. I am trying to troubleshoot, could this be a Windows firewall problem?

Created on Sep 23, 2020 12:37:01 PM



Hi Amajoura,

I would suggest you to open the support ticket for this, it's easier to do the investigate to find the root cause rather than KB here. Please provide the support bundle for further analysis.

Himanshu Bhatt (Paessler Support)

Created on Sep 24, 2020 9:54:49 AM by  Himanshu Bhatt [Paessler Support]

Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.