an operational amplifier has its inverting input connected to a electrochemical gas sensor for amplifying the current produced thereby in response to presence of a predetermined gas . in order to determine whether a sensor is indeed present and that a sensor present is serviceable , a transient is applied to the non - inverting input of the operation amplifier . the presence or absence of the sensor alters the transfer function of the operational amplifier in respect of the test signal . if a serviceable sensor is present , the gain of the operational amplifier is high for the transient resulting at square pulse output . however , if a serviceable sensor is not present , the gain of the operational amplifier is relatively low and the transient retains its original form . consequently the presence of a serviceable sensor can be determined from the output of the operational amplifier in response to the transient test signal .