PRTG 9 Manual: Sensor States

The sensors are the 'heart' of your PRTG installation. With sensors, you can monitor different aspects of your devices. Using a simple color code, they always show you what is going on with your devices.

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



Status Name





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. Another reason for this state can be a down limit set in a sensor's Sensor Channels Settings.



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.



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.




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.




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 set in the system administration.


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.

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 simply a crashed server.

After a failed request, PRTG tries to reach the device again before setting a sensor to Down state (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 state. 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 state 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 described behavior does not apply to a Warning or Down status that is activated due to a warning or down limit set in a sensor's Sensor Channels Settings.


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

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