This article explains the meaning of the message "pmbus read failed for cmd 0x8b" that is reported on Junos devices.

The following five messages are logged in syslog:

May 29 14:36:15 xyz-lab-fw1 : %PFE-3: fpc1 mpcs_i2c_single_io: MPCS(0) ctlr 3 group 0 addr 0x48 prio 0 flags 0x10 failed status 0x1
May 29 14:36:15 xyz-lab-fw1 : %PFE-3: fpc1 I2C Failed device: group 0xabcd address 0x48
May 29 14:36:15 xyz-lab-fw1 : %PFE-3: fpc1 pmbus_npcfpc_smb_write: npc_smb_write failed for group 0xabcd addr 0x48 cmd 0x0 bytes 1
May 29 14:36:15 xyz-lab-fw1 : %PFE-3: fpc1 ltc3880_pmbus_access_setup: Could not enable channel (channel 0)
May 29 14:36:15 xyz-lab-fw1 : %PFE-3: fpc1 pmbus_read_volt: summit-srx1ru-mpc - LTC3887-VDD3V3-PW-OUT: pmbus read failed for cmd 0x8b

The message is logged when the I2C primary fails to read the output voltage (0x8b) value of the I2C backup device LTC3887 voltage regulator on address 0x48 within the given timeout period due to a transient pmbus slow bus transaction, as in the given set of logs in symptoms.

This can vary wrt the card type or device type in multiple platforms.

By I2C protocol, the I2C primary will generate error messages mentioned in the 'Symptoms' section to report this failure. Then, the I2C primary will attempt a few more times. If these logs occur only once, this means that subsequent attempts succeeded in clearing the error.

Occasionally, the occurrence of these five syslog messages can be safely ignored because they do not cause any functional impact. The messages only indicate that the voltage regulator has failed to complete a software read of its current output voltage.

But if these logs continuously appear, within a span of five minutes or so, it could indicate an issue and you should open a Technical Support case for further investigation.

05-07-2023: Initial version