even before you flashed that custom rom was it able to get diagnostics port in cdma or dfs?

Usually when you flash a custom rom whether it be a htc, samsung or LG, diag codes will not work. But with LG's you have to enable debugging, put the connection mode into LG software, or modem under PC connection.

Diag port should show up under device manager and allow you to connect, if not then flash back to stock is probably the solution, I don't mess with LG motions but that should do the trick. make sure your LG drivers are up to date.