There are SNMP devices that sometimes change the OID for a specific counter. This is also common for systems running Windows 2003.

If this happens PRTG can not continue to monitor this counter and it is easiest to add a new sensor. If the historic data needs to be bound to the new sensor in order to retain the monitoring structure, please proceed as follows:

Important: please retain the old sensor until it is safe to remove it! You will need the sensor in order to discern the dataset name!

• add new sensor
• save
• edit the sensor (right-click on sensor -> edit)
• go to the "Identification" entry
• click on the "Copy ID" link on the right side
• post it in a text editor and exit
• head to the old sensor and edit it and copy the ID as well
• exit PRTG altogether (both GUI and service)
• head to your data directory (http://www.paessler.com/support/kb/questions/25)
• open the desired .PRTG configuration file with a text editor
• search for the copied ID entry of the new sensor
• replace the new ID with the old ID
• search for the old sensor and replace the old ID with the new sensor's ID
• save and exit

Now, the new sensor should be bound to the historic dataset files of the old sensor. Once you start PRTG you can delete the old sensor (please check if the new sensor is properly bound).

To recalculate the historic data of a sensor without recalculating the entire historic data of the entire configuration

• edit the sensor
• check the spike filter option
• exit the configuration window
• edit the sensor again
• uncheck the spike filter option (unless desired)
• exit the configuration window

Your historic data should now also show up in the graphs.