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There may be a harness issue, but try this first.
Short a paperclip across pins 4 and % on the computer DLC connector and see what happens.
On the connector that you attach a scan tool with, there are 16 pins. The top row 2 center pins are 4 and 5. These are computer/chassis grounds. If you short across them, you eliminate a bad ground issue, so if everything starts to work normally again, you found you issue.
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