NOVELTY - A fuel cell assembly comprises an electrochemical sensor that measures gas concentrations in interior fluid passages within the assembly or within fluid passages employed to transport reactant fluid streams to and from fuel cell(s). The sensor comprises an active electrode, passive electrode, electrolyte, substrate for the electrolyte, and a heater for heating the electrolyte.    USE - As fuel cell assembly.    ADVANTAGE - The electrochemical sensor detects a target gas with a sensitivity of 1-20000 ppm, and emits a signal representative of the target gas concentration within the sensitivity range. The sensor is insensitive to changes in the amount of oxygen and water. The sensor can measure low hydrogen concentrations.    DETAILED DESCRIPTION - A fuel cell assembly comprises fuel cell(s) having an anode, cathode, electrolyte interposed between the anode and cathode, fuel passage for directing a fuel stream to and from the anode, and an oxidant passage for directing an oxidant stream to and from the cathode; and an electrochemical sensor associated with one of the fuel and oxidant passages for measuring the concentration of a gas in a fuel or oxidant stream. The sensor comprises an active electrode (44), passive electrode (46), electrolyte (48) in contact with both electrodes, substrate (42) where electrolyte is disposed, and a heater (50) for heating the substrate and electrolyte. An INDEPENDENT CLAIM is also included for a method of fabricating an electrochemical gas sensor, comprising placing an electrolyte on a substrate having a heating element, heating the substrate and electrolyte to above 1100degreesC, placing the active electrode on the electrolyte, heating the substrate, electrolyte, and active electrode to above 900degreesC, placing the passive electrode on the electrolyte, and heating the substrate, electrolyte, active electrode, and passive electrode to above 800degreesC.    DESCRIPTION OF DRAWING(S) - The drawings show perspective and cross-sectional views of an electrochemical gas sensor.    Substrate (42)    Active electrode (44)    Passive electrode (46)    Electrolyte (48)    Heater (50)    Coating (52, 54)