If you have the codes and can accurately interpert them.... you're golden.
On the other hand, he'll be using a Tech I or a Tech II(depending on whether you're OBD1 or OBD2), which will actually let him see where the problem is originating from. Sometimes a thrown code is identified as such, but is not the actual cause/problem but a reaction to the problem.... if that makes any sense.
Sometimes $60.00 is worth the headache of trouble shooting several different cause/effects w/in the OS(pcm).
What's the problem?