Fuel gauge 2003 Honda Civic - Fuel gauge stays either at full or slowly goes down to hanlf tank mark. What is it?

The tank sensor is faulty. If you still know how much is in the tank with the gauge, then it's usually the limit switch which is part of the tank sensor..

My best guess is the fuel sending unit (it's in the gas tank and accessible by removing the rear seat).

You most likely need to replace the fuel level sending unit inside the fuel tank.

Could be the sending unit in the tank or the gauge on the dash.
A mechanic would test each component by disconnecting the harness at the tank.

The fuel meter inside the tank has malfunctioned. One option is to replace the fuel pump. Second option is to bring down the fuel tank and remove the fuel pump and check see if the metering rod was bent. Now if you are not having problems with the fuel pump. I would deal with the gauge being low and continue running the car until the time comes to replace the fuel pump.

You have a bad ground, you will have to remove the fuel tank and check the connectons on top of the tank, probably corroded. You can clean the terminals or change them.

Fuel Filter Removal & Installation To Remove:

1. Remove fuel tank unit beneath floor access panel.
2. Remove fuel filter from fuel pump/fuel gauge sending unit.

To Install:

1. Install fuel filter to fuel pump/fuel gauge sending unit.
2. Install fuel tank unit and floor access panel.

the fuel level sensor in the gas tank needs to be replaced easy way is a new fuel pump assembly

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.
   

most likley a sticking fuel sending unit in the tank, if it starts happening more consistantly you will have to replace the sending unit