Due to the principle of the lambda-probe, it is possible that there may occur slow continuous change in response of the sensor. In such a case, the lambda-probe provides gradually wrong reading of the oxygen concentration. Such situation is called discredibility. Generally, if the property changes of the oxygen sensor do not lead to a total sensor failure, it is difficult to recognize that the sensor is providing slightly wrong measurements, because from the outside viewpoint the combustion control seems to be working properly. However, as has been shown on figures 1 and 4, even the small change of the oxygen concentration may cause to move the CO concentration and efficiency out of the optimal working range