I have just had to have my 2001 Stang repaired for this problem. My mechanic had to get a smoke machine to find the leak. The SES light code refers to a large leak in the emmission system. It is refering to a leak in the fuel tank. The filler neck has a plastic insert where the cap screws in and that is where mine was leaking. For a little over $100. at your local Ford dealership you can get a replacement. That stopped my lights from coming back on.