On 2005 nissan frontier 4.0 V6 I disconnected positive battery cable to clean cable now gas gauge reads empty and tank is full of gas. what is wrong?

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 is a small float in your gas tank something like the one in your toilet tank(no I'm not kidding).Its position regulates an electrical current and based on its resistance gives a reading on your gas gauge. My best guess is this float got a bit stuck in the low position and it shock free after some time on the road.This is more common than you would think.Keep an eye on it and if it freezes in empty position or sometimes full position it will need to be replaced. Hope this helps,good luck.

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.

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

Usually if the gas gage does not work yes it is true it may be the sensor if it is electronicly controlled. the sncor will more than likley be in the tank. here is how to remove your gas tank.

1. Before servicing the vehicle, refer to the precautions section.
   
2. Relieve fuel system pressure.
   
3. Drain the fuel tank.
   
4. Remove or disconnect the following:
   
   • Negative battery cable
     
   • Fuel filler and vent hoses
     
   • Fuel tank skid plate
     
   • Fuel tank wiring connectors
     
   • Fuel supply and return lines
     
   • Fuel tank
     
   • Fuel pump assembly
     Fuel pump assembly mounting
     

To install:

1. Install or connect the following:
   
   • Fuel pump assembly
     
   • Fuel tank. Tighten the bolts to 27 ft. lbs. (36 Nm).
     
   • Fuel supply and return lines
     
   • Fuel tank wiring connectors
     
   • Fuel tank skid plate
     
   • Fuel filler and vent hoses
     
   • Negative battery cable
     
2. Start the engine and check for leaks.
   

sounds like the float isnt sitting proplery in the tank

They could have installed the wrong pump assembly. I would first check to see if the float arm is binding on something not allowing it to reach the empty position. If you take the assembly out, have the key onand work the sender up and down thru its travel and see if the gauge matches the sender position. Disconnect the wire going to the pump so it doesn't get damaged.