Service Theft System, PCM P1520 Code

[cadillacatera1997]

Hey all,
Still relatively new to the group.....Anyways I've got a 98 Seville SLS with 71,000 miles on her. The other day I was in diagnostic mode just for kicks and the code PCM P1520 was the only current code left. I have done various searches on the board and I can't seem to find what this code means/relates to. Also, my DIC comes up with the Service Theft System message when I try to use the valet key to start my car (the car will not start with this key by the way), and it came on last week when I started my car with the regular key, but has not returned since then. What am I in for with this one. I've got a couple of other questions too, If ya'll don't mind teaching me a thing or two. I purchased a bottle of NOS Super Coolant the other day (12 oz. bottle) and what I was wondering is if this is safe to use with GM's Dex-Cool? BTW, my Seville also has the infamous oil usage which is fine with me given the performance I enjoy. Thanks A lot All,
Andy