To check the status of the fans in a fan tray, issue the show chassis environment command. The output includes an entry for each fan tray, as shown in this example:
user@host> show chassis environment
Class Item Status Measurement
...
Fans Rear Fan OK Spinning at normal speed
Front Upper Fan OK Spinning at normal speed
Front Middle Fan OK Spinning at normal speed
Front Bottom Fan OK Spinning at normal speed
...
The row labeled Rear Fan reports the status of the Routing Engine fan tray and the other three rows report the status of the front fan trays. For further description of the output from the command, see the JUNOS System Basics and Services Command Reference.