That's the symptom of dirty connector plug on the back of the transmission. Mine actually said I was in gear 54 once.

If you broke a wire off inside would have the same effect.

Check the external connector first then you'd have to take off the pan to check inside. Theoretical a wire could have gotten pinched and shorted to ground

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)