chemical mechanical polishing systems and methods are disclosed . the system includes a polishing pad that is configured to move from a first point to a second point . a carrier is also included and is configured to hold a substrate to be polished over the polishing pad . the carrier is designed to apply the substrate to the polishing pad in a polish location that is between the first point and the second point . a first sensor is located at the first point and oriented so as to sense an in temperature of the polishing pad , and a second sensor is located a the second point and oriented so as to sense an out temperature of the polishing pad . the sensing of the in and out temperatures is configured to produce a temperature differential that allows monitoring the process state and the state of the wafer surface for purposes of switching the process steps while processing wafers by chemical mechanical planarization .