Device for calibrating and evaluating signals from exhaust gas oxygen probes

A heated amperometric oxygen probe comprises a heater 18 and a probe signal measuring device which applies a predetermined voltage to the probe and ascertains as an uncorrected measuring signal S_N, a value representing the current which is flowing through the probe and which is indicative of the oxygen concentration. The resistance of the probe at a given temperature increases with age so that the measured current signal may be lower than the correct value. To compensate for this effect, the evaluation device measures the internal resistance of the probe by applying an a.c. voltage to the probe from circuit 20, and controls the voltage Uh applied to the heater 18 of the probe in such a way that the internal resistance of the probe remains substantially constant. A calibration and compensation device 22 stores, in a predetermined calibration operating state of the engine, a value Uh-0 which is indicative of the heater voltage required when the probe is new, and, in the same calibration state, determines the heater value Uh-M which is currently required to achieve the said internal resistance. A correction value determination device determines, from the change between Uh-0 and Uh-M, a correction value KF for correcting the measuring signal, and correction device 23 corrects the uncorrected measuring signal S_N to a corrected measuring signal S_K.