Fuel Gauge Problem

When my fuel gauge gets to around 1/2 (empty), the Service Engine Soon light comes on and it appears the fuel gauge remains there. As soon as I fill the tank back up with gas, the Service light goes back off.

I'm hoping it's a problem with the sensor in the tank, or worse a problem with the emission system.

Any suggestions?

Please post year,make and model of your vehicle. I had to look in your profile to find out what year your truck is.
you may have 2 problems.
The fuel senders on those trucks have a coating on the sender that will wear off causing the fuel gauge to read incorrectly (it will set a trouble codebut will not turn on the SES light)
Secondly, you may have an EVAP system leak..The computer runs a test to see if the tank will hold vacum..if it doesn't, it will turn the SES light on.
Have the codes pulled and we can go from there.