The simplest reason your 2000 Nissan Sentra may have a problem communicating with the smog machine may be loose wiring within the OBD II connector or blown fuse(s). The most complicated fault could be a problem with the Engine Control Computer (ECU).
If you can purchase an inexpensive Trouble Code reader go ahead and do so. Try connecting to your Nissan Sentra's ECU via the OBD II connector. If the scanner lights up as it's connected to your Sentra this indicates there is power at the connector and the problem is more then likely with the signal wire between the OBD II connector and the ECU. If the scanner does not light up this might indicate either a loose or disconnected power or ground wire at the connector, and even the possibility of a shorted fuse. Check your Nissan's fuse box for fuses labeled DLC (Data Link Connector) and/or ECU.
The fuses should be located on the fuse block behind the coin tray. Our sources indicate Fuse 10 (10A) which is for a green wire to data link connector terminal 8 from ignition switch, and fuse 12 (10A) which is for a red/black wire to data link connector terminal 16 from battery.
