Heavy-duty (HD) Diesel engines are designed for on-road and off-road equipment with strongly varying operating conditions. In order to meet emission standards, several combustion control strategies and aftertreatment devices employing actual emission information have been developed. Nitrogen oxides (NO?) emissions are among the most constrained by regulations. In this brief, we present a NO? estimation method based on engine speed and indicated pressure measurement. A principal component analysis is performed to describe the information included in the pressure by means of a limited set of variables and L? regularization techniques are used to derive a simple and reliable estimator. The proposed strategy is implemented on a real off-road HD Diesel engine and validated on both static and dynamic standard test cycles.