Abstract: A system is provided for determining when the buildup of deposits in an exhaust line of a semiconductor wafer processing machine requires cleaning. Deposits in vacuum exhaust lines build up to where they eventually fail structurally, releasing particles that can contaminate equipment and processes. The time at which cleaning is required is often unpredictable, while frequent or early cleaning to avoid waiting too long unnecessarily reduces productivity. The invention provides for the monitoring of thermal properties on the inside of an exhaust line wall. Deposits cause changes in the monitored thermal properties. A heater and thermocouple can be used, for example, and the temperature at the thermocouple that is due to heat flow from the heater is measured. Buildups in the exhaust line affect heat flow to the sensor and are measurable as a decline in sensed temperature. Structural failure of the coating in the exhaust line leads to the eventual leveling off and fluctuation of the temperature measurement.