Why is my "service engiine soon" light is on?

I took it to O'Reilly's and they said something about fuel mixture too rich.

Ford said it was the "heated oxygen sensor". Said they had never had one of these situations before and had to look it up in old books. And said that they would order the sensor at a cost of $269 not including this visit. So probably over $300 most likely. I waited a week with no call from Ford, so I went to NAPA.

NAPA doesn't offer to hook up to your computer.

Next, I went to Auto Zone. They hooked up and said it was the mass airflow sensor at the intake/airfilter. They sold me an $8 can of cleaner specific to the "mass airflow sensor" only. I was told to disconnect my negative battery terminal. I did both terminals and cleaned them. I took the filter out and checked it. It was practically brand new. I located and sprayed the sensor. The sensor didn't have any dirt, grass, or refuse of any kind on it, that I could see. Put it all back together after the spray evaporated.

Cranked the engine and the light was off. It stayed off for a while, then came back on after about 100 miles. I don't drive much since I'm currently unemployed from being laid off.

I bought the truck from an individual who said the light had just come on. I have had it only a couple of months and only drove it to work. No long trips, no dirt roads.

The truck didn't have an owners manual in the glovebox. I called the guy and he said if it had one it would be in the truck.

There is some digital readout display on the ceiling that does not illuminate. The guy I bought it from said it was a compass, I think. Any relationship to the "service engine soon" light?

Any

Thank You