It is shown how a stable, simple and fast potentiometric EMF sensor for CO2 can be constructed based on an early idea by one of us. Whilst the gas sensitive electrode consists of Na2CO3, the reference electrode uses mixtures of SnO2 and Na2SnO3 or TiO2 and Na2Ti6O13. The activity of the elements is simply fixed by the ambient oxygen partial pressure. In this way the oxygen partial pressure cancels in the overall electrochemical reaction and the EMF measured does not depend on P-O2. Since the reaction free enthalpies of the systems chosen are such that CO2 does not interfere with Na2SnO3 or Na2Ti6O13, the reference side need not be sealed at all. Detailed results on the P-CO2- and T-dependence of the cell EMF and also results concerning the response time (similar to 1 s or even better) are presented.