If the lights in your 3-Series Security Keypad 3400 are not working then there might be something wrong with either of these
The batteries have run out
Option 1: Try checking the proximity sensor moving your hands in front of the sensor. If that doesn’t work out then recalibrate it by holding the button A. If this doesn’t work go for the second option.
Option 2: Replace the old batteries with new ones.
If this doesn’t work out either you can call up the customer care.