This article explains how to troubleshoot ​the following chassis alarm on Juniper MX devices
"SFB Voltage Sensor Out-of-Range"

This minor alarm may not necessarily be is an issue with the SFB. It can be raised due to 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 few 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.

There are incidents where voltage readings for SFB2-Bias-5V component may fluctuate and go down below the allowed range and that may cause the minor alarm to get reported, this could have been triggered due to some voltage glitch on the PSMs or SPMB affecting the SFBs or it could be due to some transient HW issue with SFB0.

If there is no PSM related alarm seen in the logs or even after PSM reseat the SFB voltage alarm still persists, you may need to reboot SFB0 in a MW which should clear the chassis alarm,

request chassis sfb offline slot X        >>>>>>> where X is the faulty SFB slot number
request chassis sfb online slot X

In case issue persists even after SFB reboot, please contact JTAC support for further assistance.