Sporadically seeing some of these logs:

fpc1 mpcs_i2c_single_io: MPCS(0) ctlr 0 group 4 addr 0x4d prio 0 flags 0x10 failed status 0x1 
fpc1 I2C Failed device: LTC3887-1:U15202 :EA1_VDD0V9R2, group 0x33 address 0x4d 
fpc1 pmbus_npcfpc_smb_write: npc_smb_write failed for group 0x33 addr 0x4d cmd 0x0 bytes 1 
fpc1 ltc3880_pmbus_access_setup: Could not enable channel (channel 0) 
fpc1 pmbus_read_volt: summit-mx3ru-mpc - LTC3887-EA1-VDD0V9R2-CH0: pmbus read failed for cmd 0x8b

MX10003 and MX10008 platforms

mpcs_i2c_single_io / pmbus_read_volt logs

If this error happens once every time, this is a false alarm, and cosmetic issues for subsequent readings are successful (no more messages are reported). There is no service impact. All the errors are related to I2C read failure on several components but the components are working. 

On MX10003 and MX10008 platforms, some false error messages (like Voltage read failed) might be seen. The reason is that its internal i2c bus may experience congestion. Thus the software may not be able to communicate on the first try. 

If the router is in good shape, You could even filter out these lines from the messages:

# set system syslog file messages match "!(mpcs_i2c_single_io)|(I2C Failed device)|(ltc3880_pmbus_access_setup)|(pmbus_read_volt: summit-mx3ru-mpc)"