Stuck on error 6

It is a rangefinder (laser and monocular type unit reading yardage in display
The only thing it now reads is err6 in dispaly when you hit the range button on the right. I tried disconnecting the battery and also replacing it and hit every concievable button combo I could think of and no help.
It happened so easily by just holding down bothe buttons while in the zip mode. Locked up with error 6 ever since. Its 11 years old and I hate to scrap it but if there is an easy solution I would appreicate it. I am capable of disassebly if necessary