referring to fig1 - 2 , an exhaust gas oxygen sensor signal simulator delay box 10 is inserted in series between an exhaust gas oxygen sensor ( ego ) 12 and a control module 14 to mimic the characteristics of aged or degraded ego sensors . the front panel of the simulator box 10 is divided into three sections with the left section having decade and vernier control knobs 16 , 18 for controlling rich to lean fuel mixture transitions . a right hand section has decade and vernier control knobs 20 , 22 controlling lean to rich fuel mixture transitions , and a center section has a control switch 23 and a ground offset vernier control knob 24 . for the present invention , the decade control knobs 16 , 20 have three active positions . referring to fig1 and 4 , the simulator 10 includes a mode switch 26 , preferably located on the front panel of the box , that , in a normal mode , electrically removes the simulator and , in an aged mode , inserts the simulator between the sensor 12 and control module 14 . thus , in the normal mode , a sensor signal from the exhaust gas sensor 12 is routed directly to the control module 14 . in the aged mode , the sensor signal is input to an operational amplifier 28 configured as a unity gain follower with resistors 30 , 32 connected between ground and the noninverting input of the amplifier 28 . the sensor signal is input at the junction of the connected resistors 30 , 32 . amplifier 28 and resistors 30 , 32 match the impedance sensor 12 would see if connected to the control module 14 thereby acting as a buffer to prevent distortion of the input signal . the output of amplifier 28 is connected to the inverting input of operational amplifier 34 through resistor 36 . a delay signal is also input to the inverting input of amplifier 34 through resistor 38 . the noninverting input of amplifier is grounded through resistor 40 . feedback resistors 42 , 44 are also connected to the inverting input of amplifier 34 while the output of amplifier 34 is connected through diode 46 to resistor 42 and through diode 48 to resistor 44 . diodes 46 , 48 are connected with opposing polarities to the output of amplifier 34 so that when amplifier 34 functions as a summing amplifier its inputs through resistors 36 and 38 are summed so that a positive output causes diode 46 to conduct producing a voltage across potentiometer 50 , on the other hand , a negative output causes diode 48 to conduct creating a voltage drop across potentiometer 52 . potentiometer 50 is preferably a wire wound resistor with a movable tap for adjusting effective resistance . the tap is moved by turning the vernier control knob 18 . similarly , potentiometer 52 has a tap that is moved by turning the vernier control knob 22 . the tap on potentiometer 50 connects potentiometer 50 to resistors 54 , 56 and 58 which , in turn , are connected to the inverting input of operational amplifier 60 . selection of one of the resistors 54 , 56 , 58 is accomplished by turning decade resistor knob 16 to the 1 , 2 or 3 position . values of the resistors are in powers of ten so that the second resistor 56 is ten times the value of the first resistor 54 , and the third resistor 58 is ten times the value of the second resistor 56 and a hundred times the value of the first resistor 54 . a resistance network contains resistors 62 , 64 and 66 connected in parallel to the noninverting input of amplifier 60 . the values of the resistors 62 , 64 , 66 are the same the values of the resistors 54 , 56 and 58 , respectively . selection of one the resistors from among the network is accomplished by turning decade resistor knob 16 to position 1 , 2 or 3 . position 1 selects resistor 62 along with resistor 54 while position 3 selects resistor 66 along with resistor 58 . resistance network 68 is identical to the resistance network containing resistors 54 , 56 and 58 and connects the tap for potentiometer 52 to the inverting input of amplifier 60 . resistance network 70 contains resistors the same as resistors 62 , 64 and 66 that are selected in the same manner except that decade resistor knob 20 is turned to select from among resistors in network 70 . resistor 72 is grounded on one end and connected to the noninverting input of amplifier 60 along with resistance network 70 and one of the resistors 62 , 64 or 66 . feedback resistor 73 completes the circuit in which amplifier 60 is configured as a variable gain inverter . there are two sets of controls for the simulator , one set for rich to lean transitions and one set for lean to rich transitions . to allow for independent delays of 1 ms to 1250 ms , each control set has two controls , a decade switch for which only the 1 , 2 , and 3 settings are used , and a vernier control which is adjusted from 0 . 0 to 0 . 92 . the relationship between τ in milliseconds and the decade ( d ) and vernier settings ( α ) is ## equ1 ## with values as indicated in the following table 1 . table 1______________________________________decade setting vernier setting milliseconds______________________________________d α τ1 0 → 0 . 92 1 → 12 . 52 0 → 0 . 92 10 → 1253 0 → 0 . 92 100 → 1250______________________________________ the output from amplifier 60 is input through resistor 74 to the inverting input of operational amplifier 76 . resistor 78 is connected between the noninverting input of amplifier 76 and ground . a capacitor 80 is connected to the inverting input and output of amplifier 76 thereby configuring it as an integrator . the output from integrating amplifier 76 is input through resistor 38 to the inverting input of amplifier 34 to be summed with the output of input amplifier 28 . the output of integrating amplifier 76 is input through resistor 82 to the inverting input of amplifier 84 which is configured as a summing amplifier . also connected to the inverting input , to be summed with the input to resistor 82 , is an input to resistor 86 . resistor 86 is also connected to the tap of potentiometer 88 that has one end grounded to provide a ground offset signal to shift the waveform up and down within a - 1 v to + 1 v window . the other end of potentiometer 88 is connected through resistor 90 to switch 23 that can switch between a ground position and positive and negative supply voltages to simulate an exhaust gas oxygen sensor failure . grounded resistor 92 is connected the noninverting input of summing amplifier 84 . a feedback resistor 94 is connected between the summing input and output of amplifier 84 . the output of amplifier 84 goes out to mode switch 26 , which , in the aged mode , delivers the output to the control module 14 . operation of the present invention is believed to be apparent from the foregoing description and drawings , but a few words will be added for emphasis . the electronic simulator box is capable of independently slowing the rich to lean and lean to rich response rates of the sensor signal . vernier control potentiometer 50 and decade control resistors 54 , 56 and 58 slow the rich to lean response rate , while vernier control potentiometer 52 and decade control resistor network 68 slow the lean to rich response rate . rich to lean transitions are identified from the trailing edge of the sensor signal waveform and lean to rich transitions by the leading edge . hego is an acronym for heated exhaust gas oxygen sensor and has a normal signal level of 0 v to + 1 v . a hego produces a high output for a rich exhaust air / fuel ( a / f ) ratio and a low output for a lean exhaust a / f ratio . one common mode of hego failure is a uniform shift in signal voltage transfer function in the negative direction , called characteristic shift downward ( csd ) as is known in the art . csd causes the hego &# 39 ; s lean voltage to be as low as - 1 v and the rich voltage to be as low as 0 v . the ground offset control of the aged exhaust gas oxygen sensor simulator box is used to shift hego waveform up and down within this - 1 v to + 1 v window , thus simulating a csd failure mode or correcting the waveform of a hego which is experiencing csd . it can now be appreciated that there has been presented a simulator which slows the lean to rich and / or rich to lean response rates of the exhaust gas oxygen sensor feedback signal independently from 1 to 1250 ms . this introduces shifts in the fuel control system away from stoichiometry , causing no x , co and hc emissions to change . the simulator box is wired in series between the exhaust gas oxygen sensor and the electronic control module . the sensor signal is modified by varying the response rate and / or by imparting a dc ground offset . there are two sets of controls ( vernier pot and decade resistance switch ) which independently vary the lean to rich and rich to lean response rates and a third set of controls ( vernier pot and polarity switch ) for varying the ground offset . a switch allows bypassing of the device completely . while the invention has been described with particular reference to an automobile fuel and emission system , it is apparent that the simulator is easily adapted to other fuel and emission systems . as is evident from the foregoing description , certain aspects of the invention are not limited to the particular details of the examples illustrated , and it is therefore contemplated that other modifications and applications will occur to those skilled in the art . it is accordingly intended that the claims shall cover all such modifications and applications as do not depart from the true spirit and scope of the invention .