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My guess is because of the exhaust. Grab some mil elim off of American muscle or get a handheld and tune it where they are turned off..
My other guess is he reset the battery before handing the car over to you :P
as far as the MIL's, I bought 2 sets and none of them worked. First one was defective and they sent me a new pair. CEL for rear o2's came back on after a full tank of gas (300 miles) Not sure if it was my car, but problem was solved after getting the sct hand tuner.