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This has been an all too common issue and a replacement cylinder has been redesigned to prevent this from happening again. The ignition cylinder will have to be replaced. What we're likely going to have to do here is open the hood and underhood fuse panel and remove one of the ignition, pcm or fuel pump fuses until the engine shuts off. Then remove the rear cargo/spare tire cover and disconnect the battery. The vehicle will require a cylinder replacement at a GM dealer. The server is down at the moment so I cannot pull up info specific to the cylinder but it falls under the been there done that many times category.