2nd Gen Prius OBD2 Port Not Communicating

First step to is to confirm its not the code reader you're using... Most code readers are counterfeit junk that don't work... I had to buy and return 3 of them before I got one that worked! Here's buyers guide: Hybrid battery diagnostic and repair tool for Toyota and Lexus

 

Could be a problem with your adapter, could be a problem with the female terminals in the OBD connector (you would need a single male terminal of the correct size to check fit when pushed in- there should be slight but noticeable drag when pulling).

Does that OBD adapter work on another car?

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I've noticed similar problems in my gen too and those code readers I tend not to use sometimes if I unplug them and reinsert them they seem like they work but I don't know how deep they go I have text stream on a cf-19 with a dongle that seems to work it's red and it lights up green and red is data moves I don't know how to buy another one to get an extra as the j standard for these things seems to be all over the place the replacement or the piece that I got with the Toyota tech stream computer I bought from the dealer I believe this is a replacement cable but it works

 

The wiring diagram shows a 7.5A OBD fuse (in the driver's side interior fuse block?) specifically for the OBD2 connector. Have you checked that fuse with an ohmmeter?