A method of assessing non-methane hydrocarbon (NMHC) conversion efficiency in a diesel after-treatment (AT) system having a diesel oxidation catalyst (DOC) arranged upstream of a diesel particulate filter (DPF) includes regenerating the AT system. Additionally, the method monitors DOC inlet and outlet temperatures during the regeneration. The method also assesses whether the DOC is operating at or above threshold efficiency by determining a DOC inlet/outlet temperature difference and comparing the determined inlet/outlet temperature difference with a threshold inlet/outlet temperature difference. The method also monitors DPF outlet temperature if the DOC is operating at or above the threshold efficiency and determines a DOC temperature/DPF outlet temperature difference. The method additionally assesses whether NMHC conversion efficiency of the DPF is at or above a threshold value by comparing the determined DOC temperature/DPF outlet temperature difference with a threshold DOC temperature/DPF outlet temperature difference. A system and a vehicle are also disclosed.