The PEFC stacks in the HCore-500 and Nexa^TM power systems were operated with room air and pure hydrogen (99.99%). Some of the fuel cells or the whole PEFC stack were separated from the embedded control system for AC impedance measurements. A 24 V rechargeable battery or another PEFC system was applied to power the electronic devices in the control system. Single MEAs, multiple MEA cell groups, and full fuel cell stacks were examined at various operational currents using an EG&G Model 273A/5210 system and a Gamry FC350^TM/TDI electronic load. The in-situ impedance data for separated cells and stacks were fitted to equivalent circuits by the non-linear least squares method (NLLS). AC impedance data were successfully measured at a high current of 35 A (ca. 291.7 mA/cm²) for the PEFC stack in its Nexa^TM system using the Gamry/TDI system without embedded system controls. PSpice simulated curves were obtained from equivalent circuit elements demonstrating good agreement with the experimental pulse data measured from the PEFC power system. AC impedance and PSpice simulation prove to be simple and trouble-free methods to implement real time diagnostic capability suitable for evaluating the peak pulse performance and the state-of-health of the fuel cells in the PEFC stack.