Replace gas cap?
 

Here's a pic of my gas cap. The rubber O ring gasket is cracked in several places and has a complete slit at the 5 o'clock position.


Is this likely the cause of the P0440 Evap emission system code I'm getting?


Can I replace the gasket with a hardware store o-ring, or is a new Saturn cap needed?

Yes for the p0440 code and no you need a new cap.
Hope that helps.
Andy

Replace the cap, clear the code and see if it comes back. I'm guessing that it won't.

What HE said.....

OK thanks, I'll get a new cap. I have a couple locking caps somewhere. I'll compare them to the Saturn and see if one fits, we'll see how that goes.

I suggest that you buy a new cap that fits your car from a parts house.

Agree. Buy a cap designed to fit your Saturn.
Use of a generic cap is asking for problems.

OK, I'll look for a Saturn cap

I'm going against the grain here on a couple of things. First, if you can get the old o-rig out, that is it's not glued in, then you should be able to replace it with a hardware store o-ring, but finding the exact size o-ring at a hardware store could be an issue.

I have a supplier on the internet for o-rings, every o-ring made, but you have to buy them 100 at a time. 100 o-rings would probably cost as much or more than a new cap.

Gas caps are universal, we all get gas from the same places and they all use the same nozzles. If you have a universal gas cap that is unvented, then I would say try it. Worse case, it doesn't work for some reason, the you could just go to a GM dealer and get a new one. But I don't see why it would not work. It does have to be a click to seal type though, a gas cap from the 60's would fit but it wouldn't seal correctly.

Edit: I just remembered that caps in the 60's were more like radiator caps, but any screw in cap that is unvented should still work.

IF you decide to try to replace the o-ring, make sure you get one that can handle gasoline. Ordinary hardware stores mostly carry o-rings intended to seal against water so check and be sure. Otherwise it'll break down quickly and you're back where you started or even worse if bits of crumbled/melted o-ring get sucked into the fuel system (seen it, not pretty).