A fuel cell system includes a stack of proton exchange membrane (PEM) fuel cells defining a body, the body including a coolant inlet and coolant outlet, a cathode inlet and cathode outlet corresponding to a cathode, an anode inlet and an anode outlet corresponding to an anode. The fuel cell system also includes a cathode humidifier fluidly connected to the cathode inlet to provide a humidified inlet stream to the cathode inlet, an oxygen sensor positioned upstream of the cathode inlet and downstream of the cathode humidifier, and configured to measure oxygen content of the humidified inlet stream, and a controller connected to the cathode humidifier and the oxygen sensor and configured to operate the cathode humidifier based on the oxygen content of the humidified inlet stream.