A method for diagnosing feedgas generation capacity of an oxidation catalyst in an exhaust aftertreatment system includes: determining a first temperature value at a first location in the exhaust aftertreatment system upstream of an oxidation catalyst, the oxidation catalyst being upstream of a selective catalytic reduction catalyst; determining a space velocity of the exhaust in the oxidation catalyst; determining an estimated exotherm value of the oxidation catalyst based on the first temperature value and the space velocity; instructing a doser of the aftertreatment system to dose hydrocarbon into the oxidation catalyst; determining an in-use exotherm value of the oxidation catalyst upon insertion of the hydrocarbon into the oxidation catalyst; and determining a fault condition based upon a comparison between the estimated exotherm value and the in-use exotherm value.