In QFX platforms each Power entry module contains a fan which when experiencing a problem will generate a minor alarm on the device.
This KB explains the steps to troubleshoot and resolve this alarm seen for  "FPC 0 PEM 0 FAN Failed".

Below alarm will be generated on the device:-

user@device> show chassis alarms
1 alarms currently active
Alarm time               Class  Description
2023-11-21 07:10:41 UTC  Minor  FPC 0 PEM 0 FAN Failed

Follow the below steps to troubleshoot "FPC 0 PEM 0 FAN Failed":-

• Check the output of "show chassis environment" to confirm all other hardware component status on the switch

user@device> show chassis environment
Class Item Status Measurement
Power FPC 0 Power Supply 0 OK
FPC 0 Power Supply 1 OK
Temp FPC 0 Sensor TopMiddle E OK 32 degrees C / 89 degrees F
FPC 0 Sensor TopRight I OK 29 degrees C / 84 degrees F
FPC 0 Sensor TopLeft I OK 23 degrees C / 73 degrees F
FPC 0 Sensor TopRight E OK 28 degrees C / 82 degrees F
FPC 0 Sensor CPURight I OK 30 degrees C / 86 degrees F
FPC 0 Sensor CPULeft I OK 29 degrees C / 84 degrees F
FPC 0 Sensor Die Temp OK 43 degrees C / 109 degrees F
FPC 0 Mezz Temp OK 22 degrees C / 71 degrees F
Fans FPC 0 Fan Tray 0 OK Spinning at normal speed
FPC 0 Fan Tray 1 OK Spinning at normal speed
FPC 0 Fan Tray 2 OK Spinning at normal speed

• Check the status of all fans on the device using "show chassis fan" output

user@device> show chassis fan
Item                      Status   RPM     Measurement
FPC 0 Tray 0 Fan 0        OK       5263    Spinning at normal speed
FPC 0 Tray 1 Fan 0        OK       5202    Spinning at normal speed
FPC 0 Tray 2 Fan 0        OK       5197    Spinning at normal speed

• Reseat the affected PSU on the device to rule out transient issues on the PSU. For the "FPC 0 PEM 0 FAN Failed" alarm reseat PSU 0 on the device
• If above does not clears the alarm try power-draining the device to rule out any issue on the chassis.
• If none of the above helps then PSU 0 will require a RMA. Please open a JTAC case for further assistance.