The easier and more reliable way to tell is to observe where your PO2 reading falls to in air after you have done your oxygen cal (I'm assuming your system uses a 1-point calibration protocol). If it falls back to somewhere substantially above .21, then you a have a more positive indication of cell failure. I'll only elaborate further if requested.

The above mentioned 6mv change can be due to a ~1.2mV (6/4.8) change in the ambient mV of your cell, easily caused by dampness, or other.

mV readings are only of any use in 2-point cal systems. They are redundant in 1-point cal systems, and 1-point cal systems are a more simple and superior method of controller calibration, IMHO.