Prop Governor Overspeed Test.
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Have you checked for a propeller governor failure in the failure interface? If that's not the problem, are you sure you are testing it correctly by holding the button while advancing the power lever, and then releasing the test button to see the RPM rise? If neither of those are the issue, then I'm afraid you will have to send me a video of the procedure you are using so I can try to duplicate the issue.
