Abstract: An internal combustion engine has an engine unit with an exhaust pipe comprising an oxygen sensor an electrical resistance heater in a bridge circuit having a control resistance and two fixed resistances RA and RB. For calibration the cell is initially operated in atmospheric air with an heater power control circuit supplying variable D.C. power output, under control of a microcontroller, to the bridge to vary the temperature of the sensor. When the sensor gives an electrical output of a predetermined reference value, this is observed by the microcontroller which varies the value of the control resistance to a calibrating value RC2 at which the bridge nulls. Then the microcontroller is instructed to adopt the operating mode and in accordance with its program the microcontroller sets the resistance value of the control resistance at the control value RC1 correlated by an operating function in the program with the calibrating value RC2.