A device for automatically controlling a by-pass electro-valve of a catalytic muffler comprising three inlet stages, the first inlet stage is responsive to the speed or number of engine revolutions, the second inlet stage is responsive to the throttle position and includes a retarding circuit and store, and the third inlet stage is responsive to the temperature of the catalyst and includes means for comparing the temperature of the catalyst with a maximum allowable temperature (TMAX.) and a predetermined running temperature (TS). The three inlet stages are electrically coupled to an outlet stage which includes a timer for producing a pulse of predetermined duration.