If the gradually increasing malfunction has not been recognized, operation of the boiler can easily be out of the optimal range. Such situation reflected in the control system of the boiler is shown on figure 6. For better illustration, there is shown sudden change of the lambda-probe functionality. There are shown responses of temperature and oxygen control loops. The temperature control loop (upper figure) is superior to the oxygen loop and controls the desired outlet water temperature. The figure shows that the malfunction of the lambda-probe causes certain instability in the output of the process; however it is soon compensated by the control loop.