I had the same problem shortly after correcting a lower intake manifold leak that had allowed oil to enter the coolant. The oil apparently fouled the sensor. I can't remember if I tried to clean it or not, but I do know that I ulitmately replaced it and the problem went away for good.
The sensor is on the passnger side of the radiator below the radiator cap. It is black and square and has a single plug at the bottom. It is held onto the radiator by a metal retaining clip. Remove the clip and the sensor pulls out of the radiator. Disconnect the wire by pulling carefully on the plastic lock and pull on the plug. Fluid will leak out to level of sensor. Drain or place catch basin below sensor plug to catch radiator fluid.
I ordered my replacement part from E-bay.
Wednesday, March 11th, 2009 AT 1:26 PM