Trailblazer 2008, Problem is while running the gears stop working

Sounds like a TCM error, but I recommend you have a transmission shop run stand alone tests on the transmission, that should tell you if it is all electronic or if there’s a valve body issue.

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The truck is fifteen years old, and in that time, automatic transmissions do go out. But your symptoms could point to something more simple, like fluid that's low and dirty, and a clogged filter. Or it might be that all the shift solenoids are going out. Have the transmission checked by a professional... It's still possible that the valve body seals are gone and pressure across the gears is doing this.