This study examines the use of open circuit (OC) auxiliary electrodes to monitor changes on the bulk potential in a sediment microbial fuel cell (SMFC). This complements the cell voltage and electrode potential measurement obtained in closed circuit (CC), helping to understand the limitations of the system and facilitating its optimization. Duplicate SMFCs were equipped with electrode grids containing integrated CC and OC electrodes. Current densities up to 20 mA/m² were obtained with the CC electrode pairs. The OC auxiliary electrodes showed a stable bulk potential around 0.7 V with cathode and anode potentials at 0.5 and −0.2 V vs SHE, respectively. A clear correlation between OC and CC potential was observed, which allowed to identify technical problems (such as oxygen intrusion to the anode or faulty electrode connections) and detect the causes of power limitation without disconnecting the CC electrodes. Thus, OC auxiliary electrodes are useful to optimize SMFC installation and facilitate troubleshooting, particularly in environments with variable bulk potentials such as intertidal zones.