The nanoprobe technique has been widely adapted to many circuit analyses from 6T SRAM bitcells to analog circuit failure analysis. But among the applications widely reported, little has focused on the flash bitcell analysis. This paper presents the more unusual case where fail device isolation is insufficient to determining the defect and/or the root cause. It provides information on measuring floating gate bitcell characteristics and understanding the fail through the customer fail mode and soft programming. In this case, the nanoprobe technique is established to understand the electrical signature of a customer return 1T flash bit fail. By using nanoprobe DC bit characterization, it was determined that data gain failures were due to thin or compromised interpoly dielectric.