I read somewhere that there is a version of the Passlock that did mention PK5, PK8 and I think I was PK4. The way I understand it the PK means it is a PASSKEY system and the number is a version. Not sure but I guess it might have to do with the sequence of rolling codes the system uses.

Is your key original or did you have it copied someplace besides GM?
Assuming your key is good, odds are good that the cylinder is the problem.

 

It probably would be the best idea to have a garage look at it and see if they can swap the cylindar.