The invention relates to a method of heating a motor vehicle cabin by means of a circuit (20) for circulating a cooling fluid for cooling an engine (10) and including a pump (23), an air heater unit (22), and a heat exchanger (21) disposed in an exhaust system (15) that is also provided with a depollution assembly (16). The exhaust gas is directed towards the heat exchanger (21) or towards a bypass duct (18) downstream from the depollution assembly (16) as a function of engine operating conditions, of the outside temperature, of the heating temperature requested in the vehicle cabin, and of the temperature of the engine cooling fluid. The invention applies more particularly to direct injection diesel engines.