A method is provided for estimating the NO x content of exhaust gas produced by an internal combustion engine. The method includes determining a first NO x estimate corresponding to the NO x level output by the engine during a first engine operating condition. The method further includes determining a second NO x estimate corresponding to the NO x level output by the engine during a second engine operating condition. The method further includes determining a compensation factor based on intake manifold pressure and applying the compensation factor to the first and second NO x estimates to arrive at a final NO x estimate.