Obd2 diagnostic reads low voltage

I have a 2000 VW Jetta 4 cyl 2.0L eng 5 sp manual. My OB2 diagnostic port isn't working it no longer allows any code readers to connect. I tried mine that use to work but now only says error malfunction check connection or replace code reader so I took it back exchanged the reader. I tried the new one and it said the same thing. I also bought a Fix code reader but it wouldn’t connect either. It says, can't connect low voltage, start vehicle and try again, yet the vehicle is already running? I tried charging the battery, I checked the wire connections to the port went throught the fuses but I can't find anything wrong to find out why it won't work. Someone told me to try and disconnect every sensor one at a time and check if the code reader is working but I didn't understand how to do that. I need to have it smogged in order to get it registered so I even took it to have it smogged and there machine wouldn't read it either so now I'm stuck does anyone have any suggestions I can't afford a mechanic I could really use some help thanks