03 chevy venture passlock lll problem

The van has passkey 3...which is transponder based. As long as you are using a programmed key the van will start, so I doubt the pk3 is the problem. What did the fob she lost look like? Was it some kind of active immobilizer ( where she had to hit a button on the fob to turn off immobilizer) or was it some kind of passive system. I would first check under the dash, and make sure there isn't some kind of starter kill hooked up, check the yellow wire coming from the key cylinder...if it's cut, then splice it back together. if not, then I would look under the hood in the main fuse box and look for any relay that is different from the others....should have a wire coming out of it.....if this is the case, then you will have to buy a relay to replace this one.