An oxygen concentration detection system includes an oxygen sensor unit sensitive to oxygen in ambient oxygen-containing gases, an activating current supply operative to supply an activating current to the oxygen sensor unit. The oxygen sensor unit is responsive to the activating current and operative to produce a sensor signal voltage variable with the concentration of oxygen in the oxygen-containing gases. A reference signal generator produces a predetermined reference signal, the activating current supply means is responsive to the reference signal for producing the activating current. A current limiting circuit limits the oxygen concentration below a predetermined value which is variable with one of the activating current and the sensor signal voltage.