A method for treating exhaust gases from an internal combustion engine in a motor vehicle, including an exhaust line provided with a selective catalytic reducer, hereinafter SCR, and with a reservoir including a reducing agent of nitrogen oxides (NOx), this method being implemented by means of a computer program executed in a computer and including the following steps: - determining an actual quantity of NOx present in the exhaust gases, - determining a quantity of NOx to be treated as a function of a predetermined quantity limit of NO x to be discharged into the atmosphere, - determining a flow of reducing agent to be injected in the SCR to eliminate the quantity of NOx to be treated, - injecting the flow of reducing agent previously determined in the SCR, from the reservoir, - modifying operating parameters of the engine to vary the production of NO x and the consumption of fuel, respectively hereinbelow low NO x emission mode and low fuel consumption mode, characterized in that the method includes, at each stoppage of the engine, a step consisting in recording in the computer the date, the time and the status of the SCR, namely if it is operating or not, then on the following start-up of the engine, if the SCR was in the operating state before the stoppage of the engine, the method includes the following steps: - calculating the time duration during which the engine remains stopped, - measuring the exterior temperature by means of a temperature sensor, - calculating the temperature drop kinetics of the SCR from the exterior temperature, - calculating the temperature of the SCR as a function of the duration of the stoppage and of the temperature drop kinetics, if the calculated temperature of the SCR is less than an operating temperature of the SCR, the computer sends data such that the engine is starting up, in low NOx emission mode, if the temperature of the SCR is greater than or equal to an operating temperature thereof, the computer sends data such that the engine on start-up is directly in low fuel consumption mode. - determining an actual quantity of NOx present in the exhaust gases, - determining a quantity of NOx to be treated as a function of a predetermined quantity limit of NO x to be discharged into the atmosphere, - determining a flow of reducing agent to be injected in the SCR to eliminate the quantity of NOx to be treated, - injecting the flow of reducing agent previously determined in the SCR, from the reservoir, - modifying operating parameters of the engine to vary the production of NO x and the consumption of fuel, respectively hereinbelow low NO x emission mode and low fuel consumption mode, characterized in that the method includes, at each stoppage of the engine, a step consisting in recording in the computer the date, the time and the status of the SCR, namely if it is operating or not, then on the following start-up of the engine, if the SCR was in the operating state before the stoppage of the engine, the method includes the following steps: - calculating the time duration during which the engine remains stopped, - measuring the exterior temperature by means of a temperature sensor, - calculating the temperature drop kinetics of the SCR from the exterior temperature, - calculating the temperature of the SCR as a function of the duration of the stoppage and of the temperature drop kinetics,