[Capri306]

If the light is on and they couldn't get ANY codes, I guarantee you they're not doing it correctly. Even if everything is 100% fine, it will STILL give you a code to say that...or maybe I'm reading your post wrong and he DID come up with a code 11/111???? Go someplace else, and post your codes here. No, Ford does not check codes for free. Any place that is has a garage is going to charge something.

Why not try pulling codes yourself? It's not that hard, and if I remember correctly, on your year of Mustang at least, you can do it with a paper clip. No, seriously, I'm not making that up. There's two spade connectors in the EEC TEST plug you can short and make the EEC go into diagnostic mode. Google it.

Just as an educated guess, I'd say it's probably HEGO (oxygen) sensors. The 5-minute drive necessary to make it come on is almost a dead giveaway.