why is speedometer not working
It's not uncommon for the speedometer cluster in the newer vehicles to fail. Often times a single resistor can burn out in the instrument cluster and cause all kinds of funky things to happen. Have a dealership diagnose whether the problem is in the speedometer assembly, the printed circuit for the cluster, or in the vehicle speed sensor where the speedometer pulls data from.
Sounds like the stepper motors are going bad. You can fix them yourself if you can solder. http://usedcarsit.com/content/chevrolet-speedometer-gauge-repair-instrument-cluster
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