You're having this issue because the mileage is stored in the BCM not the cluster for a 2009 Chevy Cobalt. Swap out the BCM as well if the part numbers match and you should be all set. Clusters and BCMs are paired to each other so they need to be from the same vehicle to work, start the car, and show the correct mileage. If for any reason it's still showing the wrong mileage, we can do a mileage correction service but you'd need to attach a note of the VIN no. to the BCM and request that we check the VIN when we receive it. Only the BCM is needed when sending it in for a mileage service on this vehicle.