A system for estimating richness of a gaseous mixture in a combustion chamber of an internal combustion engine of a motor vehicle power plant, including a mechanism injecting an additional combustible gas into a fresh air intake circuit of the engine, a mechanism measuring concentrations of the additional gas in the fresh air intake circuit and in an engine exhaust gas circuit respectively, a mechanism determining a ratio between the measured concentration of the additional gas in the intake circuit and the measured concentration of the additional gas in the exhaust circuit, and a mechanism estimating the richness of the gaseous mixture in the combustion chamber of the engine from the determined ratio.