Abstract: This invention is a self-contained NO.sub.x sensor assembly. It may be used to detect NO.sub.x levels in a flowing gas stream such as might be found in an exhaust gas from a combustion process and to produce a measurable electrical output related to the content of NO.sub.x measured. The NO.sub.x sensor assembly is of a configuration that may be detached from a mounting and replaced. The sensor assembly comprises two sensor elements one of which is made up of a catalyst on a temperature measuring device. The other is a gas stream ambient temperature measuring device. The catalyst is selected and configured so that it selectively reduces NO.sub.x and the resulting heat of the reaction raises the temperature of the allied temperature measuring device. The sensor assembly also contains a NO.sub.x reductant source. The sensor assembly may be placed in a moving vehicle for measuring NO.sub.x levels in its exhaust gas.