You didn't mention if you had replaced the battery, yet. If you haven't, then that's more than likely the problem. Also, it probably needs cleaning. You can accomplish this by using canned compressed air (like you clean your computer with). Use the straw nozzle to reach all the nooks and crannies. Clean after removing the old battery and before installing the new battery. Batteries should be changed annually and at the same time clean the unit. When using the canned compressed air, try to keep the can in the upright position, as much as possible.
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You didn't mention if you had replaced the battery, yet. If you haven't, then that's more than likely the problem. Also, it probably needs cleaning. You can accomplish this by using canned compressed air (like you clean your computer with). Use the straw nozzle to reach all the nooks and crannies. Clean after removing the old battery and before installing the new battery. Batteries should be changed annually and at the same time clean the unit. When using the canned compressed air, try to keep the can in the upright position, as much as possible.