A vehicle that includes an engine, an exhaust system, and a controller, and a method are disclosed herein. The exhaust system includes a passage for directing the exhaust gas stream from the engine through the series of exhaust after-treatment devices, including a selective-catalytic reduction device and a diesel particulate filter (DPF). The exhaust after-treatment devices are employed to reduce various exhaust emissions of the engine. The exhaust system, may however, prematurely return failing emissions results due to the amount of contaminants that have flowed through the exhaust system since the last regeneration even of the particulate filter. Therefore, the controller may, via the present method, alter the threshold of the SCR efficiency diagnostic due to contaminants accumulated on the DPF and selectively enable or disable the diagnostic based on a set of recorded instructions, to improve the robustness of the SCR efficiency diagnostic and prevent inaccurate failing emissions results.