It will usually be a fault in the keyboard membrane inside the keyboard. Most common reason is liquid spilled into the keyboard. Otherwise, there were some models that had keyboard faults and were recalled by sharp for under warranty repairs. Check with a local dealer to see if this model had the keyboard factory fault.
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The Sharp XE-A303 programming manual is here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A303_programming_manual.html
Registerman, January 2011
