In North America, heavy-duty diesel engines for on-road use have to meet strict regulations for their emissions of nitric oxide and nitrogen dioxide (cumulatively referred to as ‘NOx’) besides other criteria pollutants. Over the next decade, regulations for NOx emissions are expected to becoming more stringent in North America. One of the major technical barriers for achieving in-use NOx emissions commensurate with the levels determined from in-laboratory test procedures required by regulations is controlling NOx emissions during cold start and engine idling. Since the exhaust gas temperature can be low during these conditions, the effectiveness of the exhaust after-treatment (EAT) system may be reduced. Under colder climate conditions like in Canada, the impact may be even more significant. In the experimental study of this paper, certain engine operation parameters such as the fuel injection timing, fuel injection frequency per cycle, engine speed, and exhaust gas recirculation (EGR) ratio are evaluated for their impact on NOx emissions during idling conditions. The study is conducted on a single-cylinder, four stroke, heavy-duty research engine equipped with an intake charge cooling system and an EAT system with independent control of diesel exhaust fluid injection. In order to partially simulate a cold operational environment for the engine and highlight the associated challenges for NOx control, tests are conducted at an intake charge temperature of 10 degrees Celsius below the Federal Test Protocol mandated cold start temperature, and auxiliary cooling is used for the engine oil and coolant to maintain the temperature of these fluids to below 50 degrees Celsius. Results indicate that in-use NOx emissions can be more than an order of magnitude higher than levels determined by the present in-laboratory test procedures required by regulations immediately after cold start and during engine idling. Using a combination of medium levels of EGR, elevated idling speed and injection timing control, the gas temperature in the exhaust manifold can be ramped up to higher than 200 °C in a short duration while simultaneously reducing engine-out NOx emissions. The use of a fuel post-injection during the expansion stroke can further reduce NOx emissions at the expense of thermal efficiency.