A gas sensor assembly detects the presence of a constituent in a first gas stream. The assembly comprises: (a) a flexible circuit having a pair of oppositely-facing surfaces, (b) a sensor mounted on one surface of the flex circuit, the sensor electrically connected to conductors in the flex circuit, and (c) a channel for directing a second gas stream across the flex circuit surface facing away from the sensor, the channel formed at least in part by the flex circuit surface facing away from the sensor. The first and second gas streams can be derived from a common gas stream. In operation, thermal conductivity between the sensor and neighboring heat-conducting structures components is reduced, thereby reducing sensor electric power consumption.