A fuel cell power plant (10) having a fuel concentration sensor cell (54) is disclosed for detecting a concentration of fuel in a fuel cell (12) of the plant (10). A portion of a fuel exhaust stream is directed to flow through the sensor cell (54) adjacent to a membrane electrode assembly (60) of the sensor cell (54). A power circuit (62) may or may not deliver an electrical current to the cell (12), while changes in voltage across the cell (12) that are proportional to changes in hydrogen concentrations within the fuel exhaust stream are detected by a detector (68) which communicates the changes to a controller (108) for controlling a rate of fuel supply to the fuel cell (12). A porous sensor water transport plate (74) cools, humidifies delivers and removes liquid from the sensor cell (12).