Joule heating in flash sintering depends on the sample resistance, which is linked to the charged defects concentration. To study the contribution of defects on the flash sintering mechanism, this link should be untied. In SrTi1−xFexO3−δ (STFO) system, point defect concentration can be pinned while the resistance is influenced by the oxygen partial pressure (pO2). SrTi0.97Fe0.03O2.985 onset temperature at different pO2 was examined at constant oxygen vacancy concentration. The furnace onset temperature decreases with increasing pO2. The onset temperature of five STFO samples was predicted based on resistance-temperature dependency of the porous green ceramics in a dynamic heat balance simulation. High compatibility of model and experimental results showed reduction of onset temperature with increasing doping. Ex-situ impedance measurements of green samples reveal an overlapping Nyquist plots close to the sample onset temperature. This indicates that the onset is determined by the green body resistance regardless how it has been achieved.