'05 Alarm Goes Off After Fill-Up

[lizardth]

For some reason, when we fill the car now (or even just open the gas cap cover I recently found out), when we try to start the car, the alarm goes off and the engine won't start. We have to jump it to reset everything and then it works fine until the next time we open the gas cap cover. We got the battery tested at Wal-Mart and they said it was fine. We also took it to Autozone but their system checker did not come up with any codes. I'll be digging through the manual and continuing to search online; I was just hoping someone here might have an idea of what is going on with this... I did search all through this forum to see if this was a repost, but no luck.

This happened right after getting the 45,000 service at a different dealership than our regular one.

Edit: btw this is the factory alarm.