A system for monitoring an oxidation catalyst for a lean-burn internal combustion engine, the system comprising: a first oxygen sensor arranged to monitor an exhaust gas feedstream upstream of the oxidation catalyst; a second oxygen sensor arranged to monitor the exhaust gas feedstream downstream of the oxidation catalyst; a downstream NOx sensor arranged to monitor an exhaust gas feedstream downstream of the oxidation catalyst; and a controller (16, 18), in communication with the first and second oxygen sensors, and the downstream NOx sensor; the controller including an instruction set (17, 19), wherein the instruction set is executable to: determine, via the first oxygen sensor, a first parameter associated with O2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; and determine, via the first oxygen sensor and the second oxygen sensor, a consumption of oxygen in the oxidation catalyst due to oxidation reactions; characterized in that the instruction set is further executable to: determine an engine-out NO2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; determine a first concentration of NO2 generated by the oxidation catalyst based upon the consumption of oxygen in the oxidation catalyst; determine a second concentration of NO2 downstream of the oxidation catalyst based upon the engine-out NO2 concentration and the first concentration of NO 2 that is generated by the oxidation catalyst; determine a NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst based upon the second concentration of NO2 downstream of the oxidation catalyst and a NOx concentration measured by the downstream NOx sensor; evaluate the NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst; and detect a fault associated with the oxidation catalyst when the NO2/NOx ratio downstream of the oxidation catalyst is greater than a maximum threshold. a first oxygen sensor arranged to monitor an exhaust gas feedstream upstream of the oxidation catalyst; a second oxygen sensor arranged to monitor the exhaust gas feedstream downstream of the oxidation catalyst; a downstream NOx sensor arranged to monitor an exhaust gas feedstream downstream of the oxidation catalyst; and a controller (16, 18), in communication with the first and second oxygen sensors, and the downstream NOx sensor; the controller including an instruction set (17, 19), wherein the instruction set is executable to: determine, via the first oxygen sensor, a first parameter associated with O2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; and determine, via the first oxygen sensor and the second oxygen sensor, a determine, via the first oxygen sensor, a first parameter associated with O2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; and determine, via the first oxygen sensor and the second oxygen sensor, a consumption of oxygen in the oxidation catalyst due to oxidation reactions; characterized in that the instruction set is further executable to: determine an engine-out NO2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; determine a first concentration of NO2 generated by the oxidation catalyst based upon the consumption of oxygen in the oxidation catalyst; determine a second concentration of NO2 downstream of the oxidation catalyst based upon the engine-out NO2 concentration and the first concentration of NO 2 that is generated by the oxidation catalyst; determine a NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst based upon the second concentration of NO2 downstream of the oxidation catalyst and a NOx concentration measured by the downstream NOx sensor; evaluate the NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst; and detect a fault associated with the oxidation catalyst when the NO2/NOx ratio downstream of the oxidation catalyst is greater than a maximum threshold. determine an engine-out NO2 concentration in the exhaust gas feedstream upstream of the oxidation catalyst; determine a first concentration of NO2 generated by the oxidation catalyst based upon the consumption of oxygen in the oxidation catalyst; determine a second concentration of NO2 downstream of the oxidation catalyst based upon the engine-out NO2 concentration and the first concentration of NO 2 that is generated by the oxidation catalyst; determine a NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst based upon the second concentration of NO2 downstream of the oxidation catalyst and a NOx concentration measured by the downstream NOx sensor; evaluate the NO2/NOx ratio in the exhaust gas feedstream downstream of the oxidation catalyst; and detect a fault associated with the oxidation catalyst when the NO2/NOx ratio downstream of the oxidation catalyst is greater than a maximum threshold.