Method to detect and mitigate unsolicited exotherms in a diesel aftertreatment system

Methods for monitoring and detecting undesired exotherms which may occur in an exhaust aftertreatment system coupled to a lean burning combustion engine are described. In one particular approach, an undesired exotherm may be indicated based on an expected oxygen depletion along a length of an exhaust aftertreatment system in the direction of exhaust gas flow of exhaust gas. For example, during DPF regeneration, a certain amount of oxygen is expected to be utilized for removing soot. If less oxygen is actually found in the exhaust downstream of the exhaust system, then an undesired exotherm may be present, as unintended reductant may be present in the exhaust and reacting exothermically with oxygen. Various mitigation actions may then be initiated in response to the indication of an undesired exotherm.