A method for monitoring an oxidation catalyst that is arranged in an exhaust gas feedstream of a compression-ignition internal combustion engine, the method comprising: arranging a first NOx sensor to monitor the exhaust gas feedstream upstream of the oxidation catalyst, and arranging a second NOx sensor to monitor the exhaust gas feedstream downstream of the oxidation catalyst; determining a first relationship for the first and second NOx sensors corresponding to nitrogen oxide, NOx; determining, via the first NOx sensor, a first NOx parameter; and determining, via the second NOx sensor, a second NOx parameter; characterized in that the method further comprises: determining a second relationship (303, 305) for the first and second NOx sensors corresponding to nitrogen dioxide, NO2; determining an NO2 parameter based upon the first NOx parameter, the second NOx parameter, the first relationship for the first and second NOx sensors, and the second relationship for the first and second NOx sensors; evaluating NO2 production of the oxidation catalyst based upon the NO2 parameter; and communicating, via a controller (16, 18), the evaluating of the NO2 production of the oxidation catalyst, wherein determining the NO2 parameter based upon the first NOx parameter, the second NOx parameter, the first relationship for the first and second NOx sensors, and the second relationship for the first and second NOx sensors comprises determining NO2 production by the oxidation catalyst based upon the following relationship: NO2 DOC = 1 + Slope FTIR / SlopeNOx NO2 * NOx S1 − NOx S2 wherein: NO2 DOC represents the NO2 production by the oxidation catalyst; Slope FTIR represents the first relationship for the first and second NOx sensors; Slope NOxNO2 represents the second relationship for the first and second NOx sensors; NOxsi represents the first NOx parameter; and NOx S2 represents the second NOx parameter, wherein evaluating the NO2 production of the oxidation catalyst based upon the NO2 parameter comprises detecting a fault in the oxidation catalyst when the NO2 parameter is greater than a first threshold. arranging a first NOx sensor to monitor the exhaust gas feedstream upstream of the oxidation catalyst, and arranging a second NOx sensor to monitor the exhaust gas feedstream downstream of the oxidation catalyst; determining a first relationship for the first and second NOx sensors corresponding to nitrogen oxide, NOx; determining, via the first NOx sensor, a first NOx parameter; and determining, via the second NOx sensor, a second NOx parameter; determining a second relationship (303, 305) for the first and second NOx sensors corresponding to nitrogen dioxide, NO2; determining an NO2 parameter based upon the first NOx parameter, the second NOx parameter, the first relationship for the first and second NOx sensors, and the second relationship for the first and second NOx sensors; evaluating NO2 production of the oxidation catalyst based upon the NO2 parameter; and communicating, via a controller (16, 18), the evaluating of the NO2 production of the oxidation catalyst, wherein determining the NO2 parameter based upon the first NOx parameter, the second NOx parameter, the first relationship for the first and second NOx sensors, and the second relationship for the first and second NOx sensors comprises determining NO2 production by the oxidation catalyst based upon the following relationship: NO2 DOC = 1 + Slope FTIR / SlopeNOx NO2 * NOx S1 − NOx S2 wherein: NO2 DOC represents the NO2 production by the oxidation catalyst; Slope FTIR represents the first relationship for the first and second NOx sensors; Slope NOxNO2 represents the second relationship for the first and second NOx sensors; NOxsi represents the first NOx parameter; and NOx S2 represents the second NOx parameter, wherein evaluating the NO2 production of the oxidation catalyst based upon the NO2 parameter comprises detecting a fault in the oxidation catalyst when the NO2 parameter is greater than a first threshold. NO2 DOC represents the NO2 production by the oxidation catalyst; Slope FTIR represents the first relationship for the first and second NOx sensors; Slope NOxNO2 represents the second relationship for the first and second NOx sensors; NOxsi represents the first NOx parameter; and NOx S2 represents the second NOx parameter,