Methods and apparatuses for monitoring an oxidation catalyst in a vehicle exhaust system include a diesel internal combustion engine with an exhaust system that has an EGR, a DOC/DPF downstream from the EGR, an SCR catalyst downstream of the DOC/DPF, a first NOx sensor located upstream of the DOC/DPF, and a second NOx sensor located downstream of the SCR catalyst. A monitoring cycle of the exhaust system during operation of the diesel ICE includes executing a DPF regeneration event, shutting off the EGR, terminating urea dosing to the SCR catalyst, emptying NH3 storage in the SCR catalyst, and intrusively maintaining the temperature of the DOC/DPF within a predetermined temperature range. Readings from the first NOx sensor and the second NOx sensor allow for a ratio between first NOx sensor readings and second NOx sensor readings, the ratio providing an indication as to whether or not the DOC/DPF is failing or operating properly.