This is a pretty common issue for Trailblazers to have. Here's a link with more information about these GM gauge problems. And where you can purchase replacement parts on Amazon if you want to do the repair yourself.
Trailblazer Speedometer
Dr Speedometer Repair Kit Includes 6 Qty Six Brand...
Amazon com Dr Speedometer Repair Kit...
There is a sender unit in the gas tank something like the ball in a toilet tank.An electric current is sent through this unit and depending on the resistance will register fuel level in the tank. Simply put these wear out,you can't service them and they have to be replaced. Good luck.
You could use two resistors, Two different ohms specs for full and empty. Place resistor between proper wire colors in diagram. Turn on the key, If gage reacts properly to full and empty resistors, then I think the problem is the sending unit in gas tank. If not, the problem could be almost anywhere, wiring circuit--pcm--instrument cluster. I did see trouble codes relating to fuel gage sending unit. I hesitate to get into that? good luck
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