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Do I understand correctly that you have now replaced all four oxygen sensors? The codes tend to point to the top sensors but if the lower ones have over 100k miles, they are due as well.
First thing to check is the air path from the mass airflow sensor to the throttle body. Take that plastic and rubber air path tubing out and examine it. Also examine the tube to the idle air control valve (hard to see). While your air path is out, look to see how dirty the throttle body is. Clean the throttle body and the idle air control valve with carb cleaner spray if dirty.
You could use the carb cleaner to clean your mass airflow sensor (I have), but it is better to buy spray that is made to be MAF safe. MAF spray is not very effective at cleaning, but much safer for a MAF.
While examining the air path tubing, check the air filter box for obstruction (e.g. a rodent nest). Someone on CarTalk the other day discovered that a pack rat had been storing huge quantities of sunflower seed shells in their car.
How does the car run? If it idles smoothly and does not surge, then you probably don't have a vacuum leak.
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