How do I replace the batteries in the Code Reader?
Locate the battery cover on the back of the Code Reader and lift it off using your finger or a small coin. Replace the batteries with two AA-size batteries (alkaline-type batteries are recommended for longer life), and reinstall the battery cover. The device has a low battery warning that will flash when it's time to replace the batteries.
From the official Innova 3100 OBD2 Code Reader manual, page 7
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