Getting error code -128 on a late-2015 iMac Late 2015 iMac is rebooting on its own. Only Finder running, OSX 10.11.6. (2) 8GB RAM sticks. Got to catch it in the act today.

After reboot I find
kernel: Previous shutdown cause: -128

That doesn't appear to correspond to anything useful in the old MacOS error codes documentation from 1998:

-128  userCanceledErr         User canceled an operation

http://www.macwizard.com/errors.html

The is a refurb unit, hoping it doesn't need to go back for another refurbishment. 

thanks
The precise meaning of the shutdown cause codes is not publicly documented, so linked error about userCanceledErr isn't correct, as these are different list of codes.

The shutdown cause -128 in general is related to the unknown hardware issue (e.g. RAM). If it happens repeatedly, you should take it to the repair.

See also:


Shutdown Cause -128
Are OS X shutdown cause and sleep cause numbers listed/explained anywhere?