OBD gremlins ... need assistance please

[HSRACER]

Originally Posted by BR3W CITY

Pin 4 is chassis ground, 5 is "signal ground"
It may not be the problem, but GM does differentiate them.

The codes you posted would mostly be set to not report, so they exist on the log but don't trigger the MIL.

The "U" code is the one we're having the issues with. Thats the one actually indicating the failure on the OBD circuit. If your actually getting low voltage on the data port it means you have wires crossed somewhere. Put a meter on the data line and see if it reads anything less than 2.5v, as that's what should trigger the DTC.

Will do that tomorrow as well as relocation of the cab ground straps