diff --git a/monitor/fan.hpp b/monitor/fan.hpp
index d79ae31..9aeccd2 100644
--- a/monitor/fan.hpp
+++ b/monitor/fan.hpp
@@ -70,7 +70,7 @@
          * @brief Callback function for when an input sensor changes
          *
          * Starts a timer, where if it expires then the sensor
-         * was slow for too long and can be considered not functional.
+         * was out of range for too long and can be considered not functional.
          */
         void tachChanged(TachSensor& sensor);
 
@@ -79,6 +79,18 @@
          */
         void tachChanged();
 
+        /**
+         * @brief The callback function for the timer
+         *
+         * Sets the sensor to not functional.
+         * If enough sensors are now not functional,
+         * updates the functional status of the whole
+         * fan in the inventory.
+         *
+         * @param[in] sensor - the sensor whose timer expired
+         */
+        void timerExpired(TachSensor& sensor);
+
     private:
 
         /**