This article provides the information about replacing the SRX4200 Firewall Fan Tray

1. Identify the physical location of the faulty fan tray on the rear panel of the chassis. Figure 1 shows how the fan trays are numbered.
   Figure 1: SRX4200 Firewall Fan Tray Numbering

   
2. Attach an electrostatic discharge (ESD) grounding strap to your bare wrist, and connect the strap to the ESD point on the chassis.
3. Loosen the captive screws on the fan tray faceplate.
4. Grasp the captive screw and pull out the fan tray completely out of the chassis.
5. Insert the replacement fan tray straight into the chassis.
6. Tighten the captive screw on the fan tray faceplate to secure it in the chassis.

Refer this document for details Replacing the SRX4200 Firewall Fan Tray