I fixed this exact problem on my TI-5006 II calculator by simply replacing a "dead" battery. Three of the four battery's were good (after cleaning their terminals) but the fourth was completely bad (i.e. read "Replace" on my Battery Tester). Other problems probably can cause this exact same error message but this cure was simple and cheap. The problem first appeared after the calculator was dropped. A sudden shock (dropping it) can cause even a good battery to fail.
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