I think the pending P1456 code cleared automatically, which is what I understand the car should do if the test passed the next time it was run. I believe two consecutive measured failures are required to cause a malfunction indicator (check engine) light to be turned on.
I am still on the same tank of gas I was using on 7/2/2018 when I had the pending code, P1456. I have not touched the gas cap since it was replaced on 7/1/2018. The pending code seen on 7/2/2018 may have been set before I had replaced the gas cap (there is no way of knowing at this point). I did not try to clear that pending code (at least not intentionally).
While investigating a problem with my temperature gauge on 7/4/2018, I noticed that my standalone ODB II code reader gave me a green LED. The manual indicates that I would get a yellow LED if I had pending codes.
Today (7/5/2018) I used a Wi-Fi ODB II reader with the ODB Fusion app and on the "diagnostics" screen under the heading "pending" I now have a green check and "No trouble codes are detected." For completeness, under the heading "confirmed" I also have "no trouble codes are detected." This is the test equipment I used when I found the pending code reported on 7/2/2018 (earlier post).
I think this problem is solved, but will continue to monitor through the next tank of gas. The new gas cap seems to have been the solution.
Thursday, July 5th, 2018 AT 9:27 AM