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I'd say most likely a vacuum leak.. or a dirty Mass Air Flow sensor.
Check all vacuum hoses and connectors.. if all good spray some carb cleaner around the intake mating surface with it idling.. if you notice a change in idle, you've found your leak. Some intake leaks are very small and can be very difficult to find... sometimes you just have to replace it and hope for the best.
First though you might try removing the MAF and cleaning it... that can cause goofy things with the fuel mixture.
I am getting the same error on my 99 explorer. It started after I had the fuel system cleaned and after I reset it the check engine light comes on when I run the floor/defrost on my heater. It runs like a champ but is a pain having to reset it each time.