Gas gauge goes to empty and then full after fillup

Bad fuel sending unit.

That's got nothing to do w the O2 sensor. The float on the gas tank is a good start w the gauge issue .

Likely a bad fuel gauge (Its reading full when its only half, and when it reads half its actually on E.)

You're in luck
There is a possible recall on 2005-2008 nissan frontier/pathfinder and xterra, for erratic/inaccurate or
inoperative fuel gauge. This is the part# for the fuel gauge sending unit in question is #25060-ZS00A
Give your local nissan dealer and they should be able
to help you out. The reference number should be
#NTB07-069 AND 069A Hope this helps.

The fuel level sensor is at fault and is setting the service engine soon light on. Your local Nissan dealer can repair this under warranty depending on your mileage. The fuel tank will need to be removed to access the sending unit.

Could be the float level in tank is disconected, or fuel gauge fuse is blown, or gauge itself is malfunctioned.

It sounds like the fuel sender is bad in the fuel tank.

your gauge is part of the integrated unit in the fuel pump module

Try to check the guage sensor in your tank maybe the contacts need to be checked or may be needs to be replaced...

I just fixed this problem last weekend in my 2005 Pathfinder. It's not the fuel sending unit itself, it's the fuel level sensor that is attached to the sending unit. I'm not a mechanic, but know a little about fixing vehicles and I did it myself in about 2 hours. The dealer may tell you they need to replace the whole fuel sender, but that is false. The sensor costs $80-90 from the dealer or from Courtesy Nissan in Texas. They will ship it to you. You don't need a "special nissan tool" to remove the sending unit, unless they call a hammer and screwdriver "special". It's a simple locking ring. You could probably have any shop do it for $100 labor if you supply the part. You can find instructions if you search Nissan forums. The Frontier has the same problem (since it's the same truck with a different body) so look in Frontier forums as well. Good luck!