Fuel gage / tank issue only using 1/2 tank

[mario430]

Hello all I just got my first Diesel Truck and man do I love it. I have an 2008 GMC Sierra 2500 crew cab 4X4 short bed. According to the dealer I have a 26 gallon diesel tank but I'm only using 14 or so gallons before my gauge says I'm on empty 30 miles to go. I have not run it past that since I've heard its bad to run diesel dry. Has anyone seen this before or any idea's on what to try to fix this? Any help would be appreciated dealer wants $200 to diagnose this.

 

[mario430]

I figured it out I took it to a mechanic at the dealership and he was able to test the level sensor and he told me that it was bad. I paid him $150 and he dropped the tank and replaced the sensor.