Nellcore 595 err 6

I looked through the manual for a bit.  There are two things I would check:

-It appears there is a voltage switch on the back of the unit.  Make sure that is set according to your wall voltage.

-Check the critical battery.  It's internal.

Just me.

T.   

6 SpO2 front end reports calibration (offset) failure

Service Manual is locate HERE.

I would try a new cable and sensor, then look at the UI PCB.