Abstract: A method for operating an internal combustion engine (100) and exhaust system therefor includes the step of initiating a regeneration event in a diesel particulate filter (DPF 109) (401). A first oxygen concentration (213) of a gas upstream of the DPF 109, and a second oxygen concentration (215) of the gas downstream of the DPF 109 are sensed (403) to infer a rate of combustion of material in the DPF (405) based on the difference between the first (213) and the second (215) oxygen concentrations. The rate of combustion is compared to a threshold value (409) and the regeneration event in the DPF 109 is terminated (411) if the rate of combustion is above the threshold.