odd Dallas ds18b20 temp sensor

Hi... I just discovered that I have 2 different temperature sensors from Dallas.

on the to92 chip it reads: ds18b20p +124ae and the other ds18b20p +493af

my arduino correctly reads the +124ae but it seems confused - (I just get 0 for the reading) with the +493af..

Has anyone else delt with this problem before? I hate to chuck the +493's - I've got about 10 of them. But I also can't use them... I'm using a 4.7k resistor with both... Is there perhaps something else I am not considering?