A hardware issue with the PEM or an issue with the power supply to the PEM may generate the PEM Input Failure major alarm.

The command show chassis alarms reports the major alarm: PEM Input Failure and  PEM Not OK .  What does this mean?

lab@mx240> show chassis hardware    
Hardware inventory:
Item             Version  Part number  Serial number     Description
Chassis                                JN1128A2FAFC      MX240
Midplane         REV 07   760-021404   TS2397            MX240 Backplane
FPM Board        REV 03   760-021392   ABBA7117          Front Panel Display
PEM 0            Rev 01   740-022697   QCS0931C07U       PS 1.2-1.7kW; 100-240V AC in
PEM 1            Rev 01   740-022697   QCS0931C08L       PS 1.2-1.7kW; 100-240V AC in
PEM 2            Rev 01   740-022697   QCS0931C081       PS 1.2-1.7kW; 100-240V AC in
PEM 3            Rev 01   740-022697   QCS0931C080       PS 1.2-1.7kW; 100-240V AC in
(snip)

Only PEMs 0 and 1 are cabled. The result is these alarms: 

lab@mx240> show chassis alarms 
4 alarms currently active
Alarm time               Class  Description
2014-10-23 17:43:35 UTC  Major  PEM 3 Input Failure
2014-10-23 17:43:35 UTC  Major  PEM 3 Not OK
2014-10-23 17:43:35 UTC  Major  PEM 2 Input Failure
2014-10-23 17:43:35 UTC  Major  PEM 2 Not OK

lab@mx240> show chassis power 

PEM 0:
  State:     Online
  AC input:  OK (1 feed expected, 1 feed connected)
  Capacity:  1027 W (maximum 1027 W)
  DC output: 300 W (zone 0, 6 A at 50 V, 29% of capacity)

PEM 1:
  State:     Online
  AC input:  OK (1 feed expected, 1 feed connected)
  Capacity:  1027 W (maximum 1027 W)
  DC output: 300 W (zone 0, 6 A at 50 V, 29% of capacity)

PEM 2:
  State:     Present
  AC input:  Out of range (1 feed expected, 1 feed connected)
  Capacity:  1590 W (maximum 1590 W)
  DC output: 0 W (zone 0, 0 A at 0 V, 0% of capacity)

PEM 3:
  State:     Present
  AC input:  Out of range (1 feed expected, 1 feed connected)
  Capacity:  1590 W (maximum 1590 W)
  DC output: 0 W (zone 0, 0 A at 0 V, 0% of capacity)

System:
  Zone 0:
      Capacity:          2054 W (maximum 2054 W)
      Allocated power:   1145 W (909 W remaining)
      Actual usage:      600 W
  Total system capacity: 2054 W (maximum 2054 W)
  Total remaining power: 909 W
=========================

Here is the same situation with an SRX5800. Note that PEM2 is installed but not cabled, and the PEM3 slot is empty.

lab@SRX5800> show chassis alarms 
2 alarms currently active
Alarm time               Class  Description
2014-10-13 03:11:25 UTC  Major  PEM 2 Input Failure
2014-10-13 03:11:25 UTC  Major  PEM 2 Not OK

lab@SRX5800> show chassis power 

PEM 0:
  State:     Online
  AC input:  OK (1 feed expected, 1 feed connected)
  Capacity:  1700 W (maximum 1700 W)
  DC output: 600 W (zone 0, 12 A at 50 V, 35% of capacity)

PEM 1:
  State:     Online
  AC input:  OK (1 feed expected, 1 feed connected)
  Capacity:  1700 W (maximum 1700 W)
  DC output: 550 W (zone 0, 11 A at 50 V, 32% of capacity)

PEM 2:
  State:     Present
  AC input:  Out of range (1 feed expected, 1 feed connected)
  Capacity:  1700 W (maximum 1700 W)
  DC output: 0 W (zone 0, 0 A at 0 V, 0% of capacity)

PEM 3:
  State:     Empty
  Input:     Absent

System:
  Zone 0:
      Capacity:          3400 W (maximum 3400 W)
      Allocated power:   2340 W (1060 W remaining)
      Actual usage:      1150 W
  Total system capacity: 3400 W (maximum 3400 W)
  Total remaining power: 1060 W

This may be a hardware issue with the PEM, or an issue with the power supply to the PEM.

If a PEM was cabled, but the circuit breaker was shut off, or the PEM was not receiving any voltage for some other reason, the same alarms and show chassis power output would be seen. It is important to verify that the PEM is receiving the correct voltage (which may require an electrician) before assuming it is a hardware issue with the PEM.

See also KB26140 - Syslog message: CHASSISD_PEM_INPUT_BAD [juniper.net] .