Several years ago, I took my '86 Tbird in for its bi annual California smog test. The tech told me the gas cap was bad, and if I didn't allow him to replace right then, I would have to bring it back and pay for another smog check. It just so happened that he had a box of gas caps sitting right next to the smog machine, so I told him to go ahead and replace it, but give me back my old gas cap. As soon as I got home, I replaced the new cap with the original, drove it like that for two years until my next smog check was due. Went to a different smog check station and it passed with the old gas cap that had failed two years before. I think in addition to being a smog station, he was in the replacement gas cap business.