Apparatus and method for onboard performance monitoring of oxidation catalyst

A method of assessing performance an exhaust treatment system comprising an oxidation catalyst disposed in an engine exhaust stream, a reductant source for injecting a reductant into the exhaust stream downstream of the oxidation catalyst, and an SCR catalyst disposed in the exhaust stream downstream of the reductant source, the method comprising:

• reading the output of a gas sensor, disposed in the exhaust stream downstream of the oxidation catalyst, to determine the concentrations of NO and NO2 in the exhaust stream downstream of the oxidation catalyst, said gas sensor comprising a plurality of sensing or pumping cells, wherein a first cell is capable of producing a first output that is indicative of a concentration of NO2 alone or in combination with one or more other constituents, and a second cell is capable of producing a second output that is indicative of a concentration of NO alone or in combination with one or more other constituents, and further wherein if the first output is indicative of the concentration of NO2 in combination with one or more other constituents, the sensor comprises cells with sensing capabilities sufficient to isolate the contribution of NO2 concentration to said first output, and if the second output is indicative of the concentration of NO in combination with one or more other constituents, the sensor comprises cells with sensing capabilities sufficient to isolate the contribution of NO concentration to said second output;
• and
• generating an alert signal if either or both of the concentrations of NO and NO2 or the ratio of NO content to NO2 content in the exhaust stream downstream of the oxidation catalyst fall outside a target level or range.