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The number pad won't light up until a button is pressed (even if power is applied), as far as most models of Sentry safe are concerned.
Did the old batteries leak at all, or look corroded or browned on the ends or terminals? if so, you need to clean the terminals in the battery compartment with cotton moistened with alcohol, until they are shiny again, then allow at least 5 minutes for drying off time, then try batteries again.
Does your safe have a key lock? if so, use the emergency key on it, to open the safe. That is what emergency keys are for, to help open a safe when it has an electronic keypad and the batteries have failed.
Let me know if there are other aspects of this problem that need fixing; if I have already answered the question well enough, please rate my answer. Thanks.
Thank you for the information and the response.
The model # suggests that you do this:
When you used a 9 volt battery externally, was it like this procedure (for your model of safe):
Open the silicone cap found on the side
of the lock.
Insert a 9 volt battery into this slot.
Touch the keypad to wake up the safe.
Enter your five digit code.
Turn handle to open safe.
(DO NOT use rechargeable batteries or any other type of
non-alkaline battery, for this purpose.)
? Was this what you tried? if so, what result did you get? did the keypad light up, did the entering of your code result in the green 'proceed' light?