A method is provided for the determination of a NOx concentration value upstream of a Selective Catalytic Reduction (SCR) catalyst in a Diesel engine, the engine having an exhaust line, a Diesel Particulate Filter and a NOx concentration sensor downstream of the SCR catalyst, the method including, but is not limited to estimating the NOx concentration value upstream of the SCR catalyst as a function of at least two engine parameters, measuring the NOx concentration downstream of the SCR catalyst, calculating the difference Z(x, y) between the estimated NOx concentration value and the measured value and, in case the difference is higher than a predetermined threshold, correcting the estimated NOx concentration value with a NOx concentration correction value. The NOx concentration correction value is calculated using a correction map that correlates the at least two engine parameters and the difference Z(x, y) with the NOx concentration correction value.