Hi again, and thanks for your very helpful reply.
Also, thanks so much for your patience, as I was offline when you posted back.
Usually, the Sentry model number and serial number are on a badge-shaped sticker near the upper door hinge, but that could have fallen off or it could be located elsewhere on the safe and it's hard to see.
From your description, I believe your batteries need to be changed for new ones. Even though the low battery light does not come on, doesn't mean the batteries are not strong enough to power open the lock; the fact that the proceed light is dim, can also indicate the battery power is low.
Replace the batteries with either alkaline or lithium for electronics (lithium lasts longer) only; never use rechargeable batteries. If you don't know where the battery holder is, I'll give you instructions.
If you have a round keypad, turn the outer rim to the right just a little, from the 12:00 position to the 2:00 position and remove gently. Inside, you will see the battery holder; take that out gently and remove the old batteries, replacing with the new. Make sure there is no corrosion (rust or leaked battery acid) inside the battery holder--if you see any, carefully wipe it away with a Q-tip and some rubbing alcohol; dry thoroughly before placing in new batteries.
If you have a rectangular keypad, you will see a plastic panel under it, with a tiny hole near the left side or center; take a pen point or the end of a paperclip and put it in the hole, pressing in, as you slide the panel off to the right. Inside you will see the battery holder and follow the directions above. When finished, slide the panel back on, to the left.
I hope changing the batteries will solve your problem!
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