Ford Repair: sensors that control gas mileage, oxygen sensors, ford windstar

Question
i have a 96 ford windstar and my gas mileage recently got bad. i was told by a local mechanic that there are sensors under the van that control emissions and gas mileage he wants to change all 4 of them for over $100.00 dollars each.he said all of them are bad. my van runs perfect except for the gas mileage. what should i do, i called up a ford dealer and asked him for advice and i was told that if four sensors were bad the van would be running terrible.  

Answer
Anthony-

  With sensors it's hit or miss.  Someone can have one of the exhaust oxygen sensors (otherwise know as O2 sensors) go bad and their car runs horribly.  Another person can have all 4 fail and never even know.  If you can get the thing hooked up to a code scanner it should tell exactly what's wrong and exactly which sensors to change.  Some parts stores will do this for free...I believe Autozone does.  But once you get it scanned you'll have a much better idea of what needs to be changed.  Hope this helps.

Steve