This article explains how to troubleshoot ​the following chassis alarm on Juniper MX devices:

This minor alarm may not be is an issue with the SFB. Be sure to check logs for any PEM related issues as well.

In some cases, there may be a power supply module (PSM) outputting a voltage that is out of the range/tolerance limit for certain chassis components. This would trigger a minor alarm for the component as well as the PSM. However, in other cases the alarm for the PSM would clear, but not for the component.

show chassis alarms          
1 alarms currently active
Alarm time               Class  Description
2018-10-10 17:50:05 UTC  Minor  SFB 2 Voltage Sensor Out-of-Range

For example, there is a minor alarm for SFB 2 voltage sensor out-of-range.However, when checking the log messages, an issue is seen with the PSM module 10 which got cleared.

Oct 10 17:28:14  re0-jtac-lab chassisd[15349]: CHASSISD_PEM_VOLTAGE: Power supply 10 reports problem; check output voltage
Oct 10 17:28:34  re0-jtac-lab alarmd[16982]: Alarm cleared: PSM color=RED, class=CHASSIS, reason=PSM 10 Not OK
Oct 10 17:28:34  re0-jtac-lab craftd[15352]: Major alarm cleared, PSM 10 Not OK

After this, there was an alarm for SFB 2 voltage sensor out-of-range: 

Oct 10 17:40:01  re0-jtac-lab spmb0 cmty_sfb_voltage_check_one : sfb-03-SFB2-Bias-5V voltage outside tolerance!
Oct 10 17:40:01  re0-jtac-lab spmb0 Expected range (4500 - 5500)  read value: 5554
Oct 10 17:40:05  re0-jtac-lab alarmd[16982]: Alarm set: SFB color=YELLOW, class=CHASSIS, reason=SFB 3 Voltage Sensor Out-of-Range
Oct 10 17:40:05  re0-jtac-lab craftd[15352]:  Minor alarm set, SFB 2 Voltage Sensor Out-of-Range

Therefore, the SFB alarm is a victim and not the cause. In order to clear the alarm, physically reseat or replace the PSM module that reported the problem in the first place.