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If a keypad has some keys that do not respond, and the keypad is 19 years old, the best thing to do (before replacing the keypad!) is, to obtain some Electronic Contact Cleaner spray from a good hardware store, or an electronics supply house.
Spray a bit into the cracks around the dead keys ... not too much.
Wait overnight before trying those keys again.
If that doesn't help, the keys are dead indeed for whatever reason, and replacement of the keypad module of the keypad unit - at very least - is the advisable procedure.
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