….driving along in my Explorer I saw it right in front of me.  The little glowing message….’service engine soon’.  With those of us  with experience we know that it doesn’t mean check your oil or check your coolant….noooooooooooooo!  Anyway $656.78 later I had a brand new sensor (I don’t know what it senses).  Also I learned that there are 310 other sensors in my truck.  I am seriously considering taping over that part of my instrument panel.