Temperature sensor (located behind the rear head, near the egr pipe) - but verify that the fans work. If her a/c works, the second fan should engage with the a/c. One of the fans should also engage when you enter diagnostic mode (you know, the good ol' paper clip trick, at the aldl terminal)
remember too, the first fan shouldn't engage until the gauge reaches approximately half, then the second fan a few degrees after or if the a/c is on, or if the temperature is reaching near critical)