A modular computer system includes a fan tray comprising a number of fans, a number of slots, and sets of temperature sensors. A shelf manager stores a table that records the relationship in distance between the slots and the fans, and the correlation relationship between the slots and the sets of temperature sensors. The shelf manager monitors the temperatures of different portions of each field replaceable unit according to signals from the temperature sensors. When there is one or more monitored temperatures exceeding a preset value, the shelf manager determines which of the slots corresponds to the one or more monitored temperatures, determines the number of sensors that have detected the high temperatures, and adjusts the speed of the one or more fans adjacent to each determined slot according to the determined number of sensors.