I just spent a bunch of time on RockAuto's site looking up not just gas caps for Saturns, but also the list of applications for different gas caps.

As it turns out, there is great overlap regarding use of a single gas cap across multiple car manufacturers. However, different car manufacturers used a single cap design (for example, my 1997 SC2) for many years, but then switched to something else at different points in time:

Saturn used the design from 1991-1997
Ford used it in some capacity from 1980-2010
Dodge used it in some capacity from 1978-2002

(data from rockauto site)

The take home message being that although there is indeed a large overlap among manufacturers, not all gas caps are created equal.

For about $12, do you really want to mess with an o-ring that might not hold up the 15 yrs that the oem one did?

I had a locking gas cap in the parts bin in my garage. I took both caps and removed the gaskets, and compared them. They looked a small bit different but were the same thickness and the same i.d. and o.d.

I swapped the newer gasket to my cap and it fits very nice. I don't know if the gas cap was the reason for the OBCII code but if the code comes back it won't be from the cap