Try swapping the Nav control panels,
If the NAV receivers are Kings sometimes they don't have the oomph to clear the falgs on a self test, try a different brand, they're all interchangable, have a look at the relevant connectors at the back of the rack, even though they conn out ok you might have a bent or pushed pin making a bad contact, if all that fails go sick....
Good luck
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It flew in, it'll fly out