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If a PI tag is reading “Out of Serv”, that generally implies that the OPC Interface is not receiving data or receiving bad quality data. “Out of Serv” is a digital state from the PI Server that is translated from the OPC Server. It could be that the tag is reading a bad quality value.
As Rob mentioned, it is due to no update received from OPC server. If you are using OPC DA request you to use OPC DA client and check if tag has good/bad value on OPC server.