Loose Gas Cap Code

My 2001 Lesabre keeps indicating check for loose gas cap in the engine management screen on the left side of the dash. I have replaced the gas cap with a new one from the dealer along with cleaning and sanding the gas cap neck opening. I even put a light layer of dialetric grease on the neck opening for a better seal. I have disconnected the battery and the error meesage comes back in 1-2 days of driving. Any suggestions, I'm getting stumped on this.

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