Engine light comes on with no fauld codes

I keep getting the engine light on after numerous repairs from Tires Plus. I first had a code indicating a fuel mixture problem and Tires Plus said the problem was caused by a leaky intake manifold gasket and they repaired it. After about one hour running time from three start ups the light cam back. They then changed the air intake blower and again the light cam back in the same time duration as the fist time. They then said they were seeing a problem on one of the oxygen sensors and change it. Again the light came back on. They then had a mechanic from a Lincoln dealership come up with the dealerships test equipment and he found a bad wiring harness connecting the oxygen sensor they replaced. Again the light came on. Same mechanic from the dealer came back and found a crushed and split rubber hose elbow from the PCV valve (they showed me the hose). But again the light came back after three start ups and like all the other times except the first time there were no fault codes being displayed. Tires Plus is still working for a solution but they and the Lincoln mechanic say it is hard because there are no faults indicating a possible cause. Do you have any suggestions that may resolve my problem. Also I am not being charged anything since the first repair so the company is not taking advantage of me and they are paying for the Lincoln mechanic and his equipment. By the way the car has always run great with good gas milage before and after the light came on.