Engine light and P2096 code on 2002 PT Cruiser

:confused:My Sister in Law brought me her 2002 PT Cruiser with the check engine light on. Since I have a scanner I read the code as P2096 (Post Catalyst fuel trim system low limit). I looked at some of the O2 sensor and other readings that I could with the scanner and could not see anything terribly wrong. After I reset the light it stayed off for a 5-mile trip so I concluded it is some intermittent thing. Can anyone help me look at what might be the most likely cause? Is this a common PT cruiser problem? I'm sure she'll be back with the same trouble soon, and I'd like to be able to fix it for her.