I am currently an ABD doctoral candidate at my current Ph.D program in Experimental Psychology. Long story short, I got a C+ in a core class during my Master's program (Research Methods) back in Fall 2018 and graduated from it in December 2020 regardless of this grade. I understand the majority of programs would not count this grade (especially for a core class), but my Master's program university had a default policy for programs without a handbook (mine did not have a handbook) where a 3.0 GPA was necessary for good standing and they defaulted to the graduate school policy of being allowed up to 3 C level (C+, C, C-) grades.
Long story short, I contacted the Master's program chair and director about whether my C+ grade counted towards my Master's degree and fulfilled my statistics proficiency requirement (with a "cover story" that an auditor at my current program questioned this grade) and they said it did. Despite this, my current Ph.D program accepted my Master's in full since it was also an Experimental Psychology degree. Since my current program's handbook said someone would not be allowed to graduate with a grade less than a B-, I asked an office manager who audited degrees of those in the department whether my C+ would not allow me to graduate. I was told that my previous grades would not affect anything at all.
Despite all of this, I am concerned about what may happen when the graduate school themselves has to audit my degree before I graduate. They have a default policy to accept up to a certain number of C grades towards a doctoral degree, but I am concerned that they may use the lowest acceptable grade as a B- (specified in the Ph.D program handbook) against me potentially and not allow me to graduate. In case its important, I took my current Ph.D program's equivalent of Research Methods and got an A in it. This was not to replace my C+ grade, but to get the credit since I needed it.