This article applies as of PRTG 22

Unknown (gray) sensors

There can be various reasons why sensors show the Unknown state. See the sections below for more information on what you can check if you see permanently unknown sensors in your setup. Referring to these points will also help with debugging when you contact our support team.

Narrowing it down

Try to find a common denominator among your unknown sensors:

• Which sensors are gray?
  • All sensors
  • All sensors of a probe
  • All sensors of a specific type (for example, SNMP or WMI)
  • All sensors of a device
  • All sensors in a specific network segment
  • Various sensors in various groups, devices, probes
• Are you using a cluster?
• Is the PRTG probe service running on the parent probe system?
• Did you restart the PRTG core server service without restarting the PRTG probe service at the same time? If yes, see My sensors show an Unknown state after a PRTG core server service restart. What can I do?

Option 1: The PRTG probe service is not running

If the PRTG probe service (PRTG Probe.exe) is not running, do the following:

Restart the system

Sometimes, a simple system restart can make your monitoring system function correctly again.

Check the executables

In your PRTG program directory, right-click the files PRTG Server.exe and PRTG Probe.exe. From the Windows context menu, select Properties. On the Details tab, the Product version number must fit the one you installed with the PRTG setup file (for example, if you installed a PRTG 13 version, the version number should not show a PRTG 12 version).

Check the probe log file

In your PRTG data directory, check the Probe State (x).txt file in your \Logs\debug folder. Does it give any hints on connection or overload problems? If you find irregularities, open a support ticket.

Check the Windows Event log entries

On the PRTG core server system or a remote probe system, check the Windows Event log for entries such as access violations of PRTG services or their unexpected termination.

Option 2: The PRTG probe service is running

If you see that the PRTG probe service (PRTG Probe.exe) is running, do the following:

Check the outgoing IP address of the probe

On the PRTG core server system or the remote probe system, open the PRTG Administration Tool from the Windows start menu. Select the Probe Settings for Monitoring tab and make sure the probe uses the correct outgoing IP address to connect to the devices in your network. In most cases, the auto setting will be fine.

Note: In older PRTG versions, use the Outgoing IP Address Settings tab of the Probe Administrator program for this purpose.

Wait at least one hour

In some scenarios, for example when using a cluster or many WMI sensors, it may take a while until all sensors receive data and switch to the Up state. This is especially true with a scanning interval of five minutes or more. Please be patient and wait a little longer. Sometimes, it simply takes some time for your monitoring to (re-)start.

Create two probe state files

In PRTG, go to Setup | System Administration | Administrative Tools. In the Administrative Tools for Probes section, go to Write Probe State Files and click the Go! button. This will write probe state text files to the \Logs\debug directory of your PRTG data directory.

Wait two minutes, then click the Go! button for Write Probe State Files again.

2.1 If no probe state files are generated

If these files are not written to the PRTG data directory, the probe is either not responding or there is a communication problem. Do the following:

Restart the system

Restart your PRTG core server system or the remote probe system and try writing probe state files again.

Check the probe log file

If the state files are still not written, check the Probe State (x).txt file in the \Logs\debug folder of your PRTG data directory. Does it give any hints on connection or overload problems? If you find irregularities, open a support ticket.

Check the probe connection state

In the PRTG web interface, go to Setup | System Administration | Administrative Tools. In the Administrative Tools for Probes section, check the connection status of the local probe and, if you have any, remote probes.

Check the connection from the remote probe system

If your unknown sensors are on a remote probe, log in to the remote probe system. From this system, try to ping the PRTG core server to check if a connection is possible, and if DNS resolution and routing work correctly. Make sure that connections are not blocked by server or firewall settings. Also check firewall and virus scanner settings on both the PRTG core server and the remote probe system.

Note: In some cases, pings are intentionally disabled on the server or firewall, but connections between PRTG core server and remote probes can work anyway.

Check the number of threads of the probe

On the PRTG core server or the system running the remote probe, open the Windows Task Manager. Open the Details tab, right-click the table header and select Select columns and activate the column for Threads. Look at the number of threads for the PRTG Probe process. Also, view the processes of all users on the Processes tab. Values > 200 are conspicuous.

2.2 If probe state files are generated

If the probe state files are written to the PRTG data directory, do the following:

Check time stamps

Look at two probe state files and check if the time stamps of an unknown sensor change between two probe states.

Check the probe log file

In your PRTG data directory, check the Probe State (x).txt file in your \Logs\debug folder. Does it give any hints on connection or overload problems? If you find irregularities, open a support ticket.

Check the Probe Health sensor

In the PRTG web interface, open the Overview tab of the Probe Health sensor and check the number of open requests and messages. Open requests that are constantly > 100 or messages that are constantly > 50 should be further investigated.

Recalculate the cache

As of PRTG 14.1.9, you can recalculate the cache under Setup | System Administration | Administrative Tools.

In PRTG versions before 14.1.9, do the following: On the PRTG core server, open the PRTG Server Administrator from the Windows start menu. On the Memory Usage tab, add a check mark for Recalculate cache now, click Ok, and confirm restarting services.

Check time settings

In the PRTG web interface, check the time of the last data in the Live Data table of the unknown sensors. If a sensor contains data from the future (which means that a wrong time was set in the system), all data until this time is disregarded by the core.