I purchased a Used 5N as a backup to my new 5N2. And yup, yellow power light. I have read the serial output:
Also the battery inside is sitting at 0.5v should be 3.6v-4.5 or so. I measure it while its on and its bouncing the voltage not constant, so I wonder if the PMU error is related to this Backup Battery charging circuit? I tried to replace the battery with 2xAA batteries in series to bring up the voltage on the pins as a test to see if it would like that. No change same results.
Reading PMU Version…
PMU Version: 0.0.0x0000, Failsafe Version 3 (Variant 0x000)
Considering an Update to the PMU …
**** WARNING: PMU is currently running in FAILSAFE mode - limited functionality until it is reprogrammed ****
Unit will be reprogrammed and rebooted due to PMU in Failsafe mode
PMU is now in Failsafe Mode…Starting S-Record Dump:
Then it spits this out over and over and over:
Retry S1-record 
sendSrecord: SRec  to PMU failed [status==-2000]
Anyone know how to bypass this or “Re-program” this? Is this a chip or Software defined issue?
I know enough linux to be dangerous, so any help would be greatly appreciated!