Almost certainly that foam hose has turned to powder. For a quick check, see if the sensor will draw in cigarette smoke. I replaced the foam hose with a piece of vinyl tubing on mine. Another piece of that same crappy foam tubing is used as the evaporator drain. When that one goes, the back floor pans fill with water. I would vote for replacing that while you are under the dash.
2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg
And remember folks, Only You Can Prevent Hilliary.