This article describes the development and tests of a combined NOx/oxygen Zirconia sensor. To achieve accurate NOx and oxygen measurements under lean-burn conditions, preliminary model scale experiments with a premixed methane-air burner are carried out with current production oxygen sensors as well as with our own oxygen probe. It is found from these tests that accurate oxygen sensing requires sensor operating temperatures much lower than usually set, about 600°C. Moreover, this temperature is a good compromise for NOx sensitivity and time response of the sensor. Accordingly, a new NOx/O2 sensor has been designed and prototyped. Held in a spark plug type housing, it simultaneously monitors the NOx and oxygen content of the exhaust gas. Over the wide 02 range of interest, model scale tests of the sensor with burnt gases have shown promising agreement with laboratory analysers.