Compensating for random catalyst behavior

A method for calibrating an engine control module includes sampling a first signal from a first oxygen sensor located upstream from a catalyst. The first signal indicates an oxygen content of exhaust gas produced by an engine. The method further includes predicting a response of a second oxygen sensor located downstream from the catalyst using a model of the catalyst and the first signal and sampling a second signal from the second oxygen sensor. The method further includes determining a component of the second signal based on a difference between samples of the second signal and the predicted response. The component is due to gases other than oxygen. Additionally, the method includes calibrating the engine control module based on the component of the second signal. The engine control module controls an amount of fuel injected into the engine.