Fuel gauge stays on full

Pull the wire off fuel tank to the sender unit earth it out with ignition on.The fuel gauge should go to full slow even motion if it does replace unit in petrol tank.If it doest move you need new fuel gauge.Also before any of this i sumise you have checked fuse

Bad gauge or a bad ground check make sure wires going to gauge not grounding

That is not unusual for many vehicles. Fuel gauges are approximations only. They tend to show full too long and drop very quickly toward empty later. Nature of the beasts.

Its very simple. If the gauge is showing half full then fill the tank and see how much fuel it takes. If the amount you add is approximately 50 percent of the fuel tank capacity then you know the fuel gauge is accurate . In that case the computer is giving an inaccurate reading and should be ignored. All you need is an accurate fuel gauge - it does not matter a row of beans what the computer says if the fuel gauge is reading correctly.

Usually a sign of a faulty sending unit in the tank.
A mechanic would ground the wire to the sender to see if the gauge moves.

your car probably needs a code scanner OBD2 they are called - there will be a plug for this somewhere under the steering wheel just above the pedals somewhere - Scan for any errors first before moving on to the fuel sender unit which it could also be - could also be corrosion on wires that run from the fuel sender unit itself causing resistance

Normal. There is a resister in the fuel level circuit that slows down fuel level readings. A gauge that responded instantly to a sloshing fuel level every time you went over a bump or around a turn would drive you crazy.

Most likely the fuel sending unit in the tank,but to test it unplug the unit from the tank,the gauge should go to full,if it does not,then the sending unit in the tank is bad.

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

I had the same problem. My battery was going bad. When I replaced it with a new one, the fuel gauge worked fine.