One problem I had with my sender (the same one you have from NAPA) is that a week after installing it, the bottom portion of it came loose and dropped to the bottom of the tank and shorted to ground. This gave a full reading.
I pulled it and replaced the kind of cheap looking top plastic mount between the mounting surface and stem, which the mounting bolt head had pulled through with a better fiber spacer and washer from the hardware store. The bolt that holds the sender together is also the same one that the wire attaches to on top of the sender. When you tighten the nut, it pulled the bolt through. That was the only problem I had with it. It works perfectly now.
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