The fan tray is attached to the back of the cable management system and installs into the front of the chassis, as shown in Figure 1. There are also three separate, non-interchangeable impeller assemblies. The front impeller is located behind the craft interface (shown in Figure 1) and works together with the fan tray to cool the FPCs and midplane. The upper and lower impellers on the rear of the chassis (shown in Figure 2) work together to cool the components that install into the rear of the chassis.
To check the status of the impellers and the fans in the fan tray, issue the show chassis environment command. The output includes an entry for each impeller (which it refers to as a Blower) and for each fan in the fan tray:
user@host> show chassis environment
Class Item Status Measurement
...
Fans Rear Bottom Blower OK Spinning at normal speed
Rear Top Blower OK Spinning at normal speed
Front Top Blower OK Spinning at normal speed
Fan Tray Rear Left OK Spinning at normal speed
Fan Tray Rear Right OK Spinning at normal speed
Fan Tray Front Left OK Spinning at normal speed
Fan Tray Front Right OK Spinning at normal speed
...
For further description of the output from the command, see the JUNOS Internet Software Operational Mode Command Reference: Protocols, Class of Service, Chassis, and Management.