Why does my receptacle tester say open ground on my new outlet?
Assuming you didn't reverse the hot/neutral, which should be detected on a standard receptacle tester, it's either a bad receptacle, bad wiring, bad connection between the wiring and the outlet, or a bad tester.


If it's a new receptacle, odds are low that it's bad. Test for continuity from the ground screw to the ground pin inside the receptacle (with the power turned off).
If it's a bad connection, you can do the same test as above, but from the ground wire instead of the receptacle ground screw.
If it's bad wiring, then with the breaker turned on, and with extreme care not to electrocute yourself, a contact tester will not show current between the hot and ground wires.
If none of these tests show a problem, it could be a bad tester, and may be worth checking it in other receptacles or getting a replacement.