How do I get my vehicle out of diagnostic mode if I've already disconnected the ground cable once
Try pulling the ECM fuse for just over 1 minute. This should reset it back to factory settings. It should be a blue 15A fuse.
If you have already pulled a battery cable then the ECM has been reset, just make sure you have removed the jumper for reading the flash out codes before attempting to drive it, there will be a relearn that the ECM will have to go through be just drive as you normally would and it will learn.
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