Hello.It sounds as if your float isn't floating any more.First check and make sure that the gauge itself is working, which it is, but check anyway.Unplug the wire drom the sending unit on at the front of the gas tank and plug a long wire into the sending unit wire and ground the other end of your wire to the body of the car somewhere,turn the key on and the gauge should go to full.If that checks out you need to replace the float on the sending unit. Drain the tank in a safe manner, remove the sending unit lock ring by rotating it counter-clockwise about 15 degrees, pull it out of the way and then snatch the sending unit out of the gas tank.You will see a brass float on the end of it.Pull that off and put on a new one. Reverse the process to reinstall the sending unit and you should be good to go.Hope that helps.
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