Well I know for a fact, at least for the turbo guys, that this CEL is caused by a valve right between the gas tank, and the spare tire well, under the car. There is a rubber diaphram inside it, and can tear if it sees boost pressure. In the center of this diaphram, is a plastic pole piece, with sharp edges on it, on the underside of it, and if boost pressure gets in it, can tear the diaphram on those edges. Someone on here figrured it out, and then I took mine out, took it apart, and that was it. You can file down those edges on a new valve, and put it back together, and its fixed. This valve will have like 3 vac lines coming to it. You cant miss it really. Its a bitch to get off though...