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Hardware Installation Guide for the Cisco 4000 Series  Integrated Services Router
OL-32185-02
Chapter 1      Overview of Cisco 4000 Series ISRs
  Hardware Features of Cisco 4000 Series ISRs
Fans, Ventilation, and Airflow
Chassis Ventilation
Router and chassis temperature is regulated with internal fans. An onboard temperature sensor controls 
the fan speed. The fans are always on when the router is powered on. Under most conditions, the fans 
operate at the slowest speed to conserve power and reduce fan noise. When necessary, the fans operate 
at the higher speeds under conditions of higher ambient temperature. To replace Cisco 4451  ISR, 
Cisco 4431 ISR, and Cisco ISR4351 fan trays, see the “Replace a Fan Tray” section on page 6-53.
Figure 1-26 shows Cisco 4451  ISR airflow, Figure 1-27 shows Cisco 4431 ISR airflow, Figure 1-28 
shows Cisco 4321 ISR airflow, and Figure 1-29 shows Cisco  4221  ISR airflow. 
Figure 1-26 Cisco 4451 ISR Airflow
Figure 1-27 Cisco 4431 ISR Airflow