You have a gross (vs a small - P0442) leak in the evaporative emissions system. The computer commands that a vacuum be pulled in the fuel tank, then measures the negative pressure in the tank to be sure that the vacuum is present, is correct, and that it holds. In the case of a large leak, it cannot pull a vacuum so the PCM sets the P0455 code. It also means that the test has failed on two consecutive test cycles.
The fuel cap is the only thing that a typical owner can access. Check to make certain it's tight. As least this AutoZoner got this right, heck, even a blind dog finds a bone occasionally.
Gas treatment will have no effect, much like chicken soup when you're ill. If the fault still exists, the CEL will come back eventually, anywhere from a few days to a few weeks.
If this was the result of a state-mandated emissions check, they should have also tested your gas cap. If not, you can try swapping it.
If you determine that the problem is not the gas cap, your best recommendation to locate the repair is to hire a shop that has a smoke generator to locate the leak. Likely fault locations are the CV solenoid, canister, fuel tank filler grommet, one of the seals on the top of the tank, one of the many hoses in the system, the purge valve, or an electrical fault with the pressure sensor or one of the solenoids.
Steve
