awebsurfin

When I plug in my OBD scanner, which does work correctly, everything lights up and it goes through a series of trying to connect and finally it comes back and says communication error. It’s on 801 Chevy Silverado with a 5.3. cigarette lighter fuse and all that is good and all those ports work fine.
but with obd ll connected, if I start the truck it starts like should but immediately shuts itself off. But with the OBD not connected the truck starts and runs fine as usual.
ideas?