Our fleet of iVent 201’s were having a lot of O2 issues even after O2 cell replacement and successful calibration. What I ended up doing was replacing the wiring harness that goes from the main board to the O2 cell, part# M1161986, I replaced this cable on all 11 of our iVent 201’s and it helped a lot.
I also found the best way to calibrate the O2 is to put the filter in a nitrile glove and then install it in the muffler as a way to seal the inlet very well. If it still won’t calibrate I’ve found that the muffler sometimes isn’t properly sealed up (new out of the PM kit box) and isn’t air tight, so you have to re-seal it (not recommended by GE), or get it replaced by GE under warranty.