Turns out the P1641 code is actually related to the A/C. I had noticed my air conditioner had quite working a few months back, but I chalked it up to being old
The fuse was burned through. I replaced it and now the A/C works. I don't have a code reader, so I don't know if it cleared that code, but I bet it did.
I'm left with the P0440, which is probably a gas cap problem as I've read. I had an Aztec that had a similar problem. I'll buy a new one and see what happens.