PRTG Manual: Sensor States

In PRTG's device tree you usually create several sensors on each 'device'. With sensors, you can monitor different aspects of your devices. Using a simple color code, they always show you what is going on in your network.

The color of a sensor always shows its current status. Following, you find a list of states a sensor can show.



Status Name





  1. PRTG is not able to reach the device or the sensor has reached an error state. Please see Sensor Warning and Down Behavior below for more information. Note: By design, a sensor does not record any data in its channels while it shows this status.
  2. Another reason for this state can be an error limit set in a sensor's Sensor Channels Settings, or an error status due to a sensor Lookup. Note: In this case, the sensor continues to record data in all sensor channels although the sensor shows an error.



Down (Partial)

In a cluster, at least one node reports this sensor as Down, while at least one other node reports the same sensor as Up.




There was an error reading the sensor, but PRTG will try again. The sensor may soon change to a down status. Please see Sensor Warning and Down Behavior below for more information. Another reason for this state can be a warning limit set in a sensor's Sensor Channels Settings.



Down (Acknowledged)

The sensor is Down and the status was acknowledged by a PRTG user, applying the Acknowledge Alarm function. This can be helpful to mark that an alarm has already been attended to. For acknowledged alarms no more notifications are sent. To set this sensor status, right-click on a sensor in a Down status and from the context menu, select Acknowledge Alarm.... Then enter a comment and click OK.




The last check was okay and the sensor receives data.




The sensor is currently paused (for a certain time, or unlimitedly).




The sensor reports unusual values for this weekday and time of day. The unusual detection is based on the historic average data and can be  configured or disabled in the system administration. You can also disable unusual detection for certain groups only (see Group Settings).


Black (Gray)


The sensor has not been checked yet by PRTG or there is an error in (network) communication, likely on the probe system. If sensors show this state persistently, a PRTG restart may be necessary. For extended trouble shooting please see More section below.


Sensor Warning and Down Behavior

The Down status symbolizes that something is wrong with a monitored device. There can be various reasons for a down status, e.g. an interruption in the physical connection to the device, an internet connection outage, or a crashed server.

After a failed request, PRTG tries to reach the device again before setting a sensor to Down status (this is true for almost all types of sensors):

  1. If a request to a device fails for the first time, the sensor is set to Warning status. PRTG repeats the request and tries to re-scan the device immediately.
  2. If also the second request fails, the sensor is set to Down status until the device is reachable again. PRTG tries to reach the device with every scanning interval.

This procedure gives devices and services the chance to recover from a momentary overload and prevents false alarms. Still, you are informed promptly about any failures occurring.

Note: The behavior described above does not apply to a Warning or Down status that is activated due to a warning or error limit set in a sensor's Sensor Channels Settings.


Knowledge Base: What to check if sensors are black (gray)?



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Keywords: Down,Down (Acknowledged),Down (Partial),None,OK,Paused,Sensor,Sensor Behavior,Sensor Status,Unknown,Unusual,Warning

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