Methods for monitoring the performance of an oxidizing catalyst device are provided. Methods can include treating an exhaust gas stream with the oxidizing catalyst device, determining a reference liberated oxygen (LO) species measurement of the exhaust gas stream, measuring a downstream LO species measurement of the exhaust gas stream using a NOx sensor in a catalyst inactive mode, and determining a LO species differential. The downstream NOx sensor can comprise an amperometric sensor and include a NO2 selective reduction catalyst. Methods for using an amperometric NOx sensor utilizing an NO2 selective reduction catalyst are also provided, and include operating the NOx sensor in a catalyst active mode to generate a first LO species measurement, operating the NOx sensor in a catalyst inactive mode to generate a second LO species measurement, and comparing the first LO species measurement to the second LO species measurement to determine a LO species differential.