My fuel gauge shows empty with a full tank of gas 2005 Nissan x terra

Either the fuel tank sender in the tank is bad and needs replacement, or the fuel gauge is faulty. I would bet the one in the tank is probably bad, because that's usually the case. Find the connection at the fuel tank, disconnect it and use a VOM to get a resistance reading while the tank is empty and reading 1/2. It should match the specs.

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if you have 2 fuel gauges working of a tank switch there may be some resistance in the circuit of the second circuit that increases the reading of the gauge. Personally I have the tanks balanced out with a levelling hose between them the the gauge reads the capacity left in the tanks IE full-- both tanks-- full, 1/2 both tanks 1/2 With the way you have it set up then if one gauge is faulty and shows full when it is empty you can find yourself in a spot of bother when you need the other tank

2005 PT cruiser, i replaced the cam sensor with a aft market sensor my fuel gauge started reading wrong, and it didn't fix my problem so i replaced the cam sensor with a factory sensor it fix my running problem but the fuel gauge still reads wrong, with a full tank of gas the gauge reads 3/4 tank,

There may be an updated sensor to install refer to service bulletin 07-069A your dealer can find it for you.

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.

The oxygen sensor has nothing to do with the fuel gauge. There are two possibilities for the gas gauge error, first it could be the in-tank gas level sensor or it could be a defective stepper motor which drives the gauge. A professional scan tool can determine the fuel level being sent by the in-tank sensor, if that disagrees with the gauge, then the stepper motor needs replacing.

Sending unit died. can be expencive to replace since most sending units are integral with the fuel pump now. lots of labor too. Better to use the trip odometer till you can afford a day without your car and a $600.00 +labor repair bill.

u need to change this gauge or try to dissasseble and clean it very well then try to install it back again