2001 Lincoln Continental The 'Service Engine Soon' light ke

As I said above, the 'Service Engine Soon' light keeps coming on. And my mechanic buddy has put the diagnostic computer on it several times, and it always comes back with a code pointing to the gas-cap. He rests it, and a couple of weeks later, it's on again. We've checked the gas-cap, nothing wrong, even tried putting on a new cap, still have the problem. My buddy was a Lincoln Service Manager for many years, so he knows what he's doing, but he's baffled by this too. So I thought I'd ask you guys if you've ever run across this problem before?
Thanks,
RockinRocky
rockyf@comcast. Net