Methods of and apparatus for monitoring the health of a fuel oxygen conversion unit for a vehicle or a vehicle engine are provided. For example, a method comprises providing a distribution of sensed values obtained from a plurality of sensors distributed along an axial length of a gas oxygen reduction unit of the fuel oxygen conversion unit and determining the health of the fuel oxygen conversion unit from the distribution. As another example, a fuel oxygen conversion unit defines a circulation gas flowpath from a fuel gas separator to a contactor and comprises a gas oxygen reduction unit positioned in the circulation gas flowpath for reducing an oxygen content of a flow of stripping gas through the circulation gas flowpath. A plurality of sensors are distributed along an axial length of the gas oxygen reduction unit.