2005 Buick Lesabre

Hi Bob, In our experiences with fuel cap replacements we like to use OEM factory caps to ensure a proper seal. A couple things to inspect if the cap is not the case of the problem. On the fuel tube that goes down to the gas tank where you place the cap is there any damage to it making it not able to seal properly, sometimes these things can get damaged while putting gas in your car time after time. Another problem could be your down tube to the gas tank could half slight rips or holes causing the leak of pressure for that gas tank sensor. But first lets take the easy route replace the cap with an OEM factory cap and get back to me if the problem is still present.

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Another thing to consider would be that if there isn't any leak then the sensor in the gas tank could be bad.