A major alarm occur with bottom fan tray failure error. 

> show chassis alarms 
1 alarms currently active
Alarm time               Class  Description
2023-11-11 11:38:57 AEDT Major  Bottom Fan Tray Failure
 
 alarmd[25560]: Alarm set: Fan id=16777572, color=RED, class=CHASSIS, reason=Bottom Fan Tray Failure

 craftd[23042]: Major alarm set, Bottom Fan Tray Failure

Bottom Fan try failure with below message log:
 

Nov 11 11:38:57.845 2023 alarmd[25560]: Alarm set: Fan id=16777572, color=RED, class=CHASSIS, reason=Bottom Fan Tray Failure

Nov 11 11:38:57.845 2023 craftd[23042]: Major alarm set, Bottom Fan Tray Failure

Nov 11 11:38:59.351 2023 chassisd[23038]: CHASSISD_SNMP_TRAP6: SNMP trap generated: Fan/Blower Failed (jnxContentsContainerIndex 4, jnxContentsL1Index 2, jnxContentsL2Index 1, jnxContentsL3Index 0, jnxContentsDescr Bottom Tray Fan 1, jnxOperatingState 2)

Nov 11 11:38:59.352 2023 chassisd[23038]: CHASSISD_SNMP_TRAP6: SNMP trap generated: Fan/Blower Failed (jnxContentsContainerIndex 4, jnxContentsL1Index 2, jnxContentsL2Index 2, jnxContentsL3Index 0, jnxContentsDescr Bottom Tray Fan 2, jnxOperatingState 2)

Nov 11 11:38:59.352 2023 chassisd[23038]: CHASSISD_SNMP_TRAP6: SNMP trap generated: Fan/Blower Failed (jnxContentsContainerIndex 4, jnxContentsL1Index 2, jnxContentsL2Index 3, jnxContentsL3Index 0, jnxContentsDescr Bottom Tray Fan 3, jnxOperatingState 2))

Nov 11 11:39:03.896 2023 chassisd[23038]: CHASSISD_BLOWERS_SPEED_FULL: Fans and impellers being set to full speed [fan/blower missing/failed]

user> show chassis environment 

Class Item              Status   Measurement

Temp PEM 0             OK     25 degrees C / 77 degrees F

   Bottom Tray Fan 11       Check   

Suggest Customer to clear fan tray and filter, Reseat the bottom fan tray and if the alarm still exist, RMA should be raised to replace the bottom fan tray.

Referenced KB34415 [juniper.net].
[MX960] Bottom and Top Tray Mappings to Fan Tray Numbers

Fan Tray 0 is the Upper Fan Tray.

Fan Tray 1 is the Bottom Fan Tray.