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,

SOURCE: Fuel Gauge Not Reading

you might wanna check the contact on the floater i had that problem so i just bent the contacts closer and that worked for me

SOURCE: gas gauge reading empty even when tank is full

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
     

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.
   

SOURCE: gas gauge reads empty but tank acts like it's full

I also have a 04 with a 1.8L. same problem. I took the fuel regulator off. It is a black box behind the rear tire under car. It only has 2 bolts to take off. After that there is a couple of vent hoses from the tank going to this box. Unplug them and 1 electrical connection. The electrical connection and one vent is connected to a T connection on the box that you can unplug by twisting on the T. Take that T off and look inside where you unplugged the hose on this T. There is a spring and a little plunger inside this tube. The plunger gets stuck. I took a little screw driver and some PB blaster and worked that plunger back and forth a couple dozen times and it unfroze. Put it back together gassed up just fine. I did not look but I was told regulator can be replaced for 200$. But a can of blaster is cheaper.

SOURCE: 2002 Chevy Suburban: Fuel gauge reads empty, tank is half full

Youre gauge is controlled by a plunger in the fuel tank which is attached to the sensor and the fuel gauge. it sounds like when the plunger goes down half way its either getting stuck down lower then it should. Needs replacing. Any mechanic can do this install for you.

SOURCE: my 2001 Pontiac Aztek's fuel gauge works until

it is one of two things the fuel gauge in the tank is bad or the gauge in the dash is bad...I would check with the dealer to see if they have seen that problem before...they maybe a recall for that problem...