MIB: JUNIPER-MIB
If they are using DC power supplies there is a known issue on PR 1064039 where the fans do not detect the temperature correctly and fail to cool the power supply causing the shutdown to occur.
This is fixed in Junos 13.3R7 https://forums.juniper.net/t5/Routing/PEM-0-not-OK-MX104/m-p/289644#M14122