A key is failing to be read when it is pressed. This can happen because of mechanical failure within the switch element, or from a broken or mis-wired keypad wire. If, however, the problem is with a wire, more than one key will be "dead". If just one key is not being echoed, the problem is with the key itself. You can, of course, use the unit for all operations that don't require that particular key but you will want to replace the keypad for full operation.