System for Detecting Deterioration of HC Sensors for Internal Combustion Engines

This invention relates to a system for detecting deterioration of HC sensors arranged in the exhaust passage of an internal combustion engine for detecting concentration of hydrocarbons (HC) in exhaust gases emitted from the engine.

Conventionally, a system has been proposed by Japanese Provisional Patent Publication (Kokai) No. 50-47228, which uses an HC sensor arranged in an exhaust passage of an internal combustion engine, to control in response to output from the HC sensor, an amount of fuel and an amount of air supplied to the engine such that the concentration of noxious components (HC) in exhaust gases decreases to the minimum value.

Further, a system for detecting deterioration of a three-way catalyst of an internal combustion engine has been proposed by the present assignee e.g. by U.S. Ser. No. 07 717,247 filed Jun. 18, 1991. The proposed system uses two HC sensors arranged in an exhaust passage of an internal combustion engine respectively at locations upstream and downstream of a three-way catalyst arranged in the exhaust passage, to determine whether the three-way catalyst is deteriorated or not, by comparing outputs from the HC sensors.

However, in general, the performance of HC sensors such as an output characteristic thereof deteriorates due to aging etc. If various controls are carried out based on the output from an HC sensor which is thus degraded in performance, such controls cannot attain required control accuracy.

More specifically, in the above described two systems, if the output from the HC sensor which does not accurately reflect the concentration of HC in exhaust gases is used, accurate air-fuel ratio control cannot be effected, or accurate detection of deterioration of the three-way catalyst cannot be effected.

It is an object of the invention to provide a system for detecting deterioration of at least one HC sensor of an internal combustion engine, to prevent degradation in accuracy of a control based upon the output from the at least one HC sensor, and assure accurate detection of deterioration of a catalytic converter arranged in the exhaust pipe of an internal combustion engine.

To attain the object, the invention provides a system for detecting deterioration of at least one HC sensor provided in an internal combustion engine having an exhaust passage, the at least one HC sensor being arranged in the exhaust passage for detecting concentration of hydrocarbons present in exhaust gases from the engine.

The system according to a first aspect of the invention is characterized by comprising:

difference-calculating means for calculating a difference between an actual value of output from the at least one HC sensor and a predetermined reference value set in accordance with at least one parameter indicative of operating conditions of the engine, while the engine is in a predetermined operating condition;

comparison means for comparing the difference with a predetermined value; and

judging means for judging that there is deterioration in the at least one HC sensor when the difference exceeds the predetermined value.

Preferably, the at least one parameter indicative of operating conditions of the engine is an amount of fuel supplied to the engine.

More preferably, the predetermined operating condition of the engine is cruising of a vehicle on which the engine is installed.

Further preferably, the predetermined operating condition of the engine is feedback control of an air-fuel ratio of a mixture supplied to the engine.

Still more preferably, the engine includes a catalytic converter arranged in the exhaust passage for purifying exhaust gases from the engine, the at least one HC sensor comprising an HC sensor arranged in the exhaust passage at a location upstream of the catalytic converter, the actual value of output from the HC sensor being a learned average value of output from the HC sensor.

Preferably, the engine includes a catalytic converter arranged in the exhaust passage for purifying exhaust gases from the engine, the at least one HC sensor comprising an HC sensor arranged in the exhaust passage at a location downstream of the catalytic converter, the at least one parameter indicative of operating conditions of the engine being an amount of fuel supplied to the engine and a temperature of the catalytic converter.

More preferably, the predetermined operating condition of the engine comprises a condition in which the temperature of the catalytic converter is lower than a predetermined value.

Preferably, the judging means judges that there is abnormality in the at least one HC sensor when the difference has continued to exceed the predetermined value over a predetermined time period.

The system according to a second aspect of the invention is characterized by comprising:

memory means for storing a value of output from the at least one HC sensor assumed when fuel supply to the engine is cut off;

correcting means for correcting an actual value of output from the at least one HC sensor by the value of output from the at least one HC sensor stored by the memory means to obtain a corrected value of the actual value of output from the at least one HC sensor;

difference-calculating means for calculating a difference between the corrected value and a predetermined reference value set in accordance with at least one parameter indicative of operating conditions of the engine, while the engine is in a predetermined operating condition;

comparison means for comparing the difference with a predetermined value; and

judging means for judging that there is deterioration in the at least one HC sensor when the difference exceeds the predetermined value.

The above and other objects, features, and advantages of the invention will become more apparent from the ensuing detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram showing the whole arrangement of a fuel supply control system of an internal combustion engine including a system for detecting deterioration of HC sensors according to the invention;

FIG. 2a, 2b and 2c show a flowchart of a program showing the manner of detecting deterioration of the HC sensors, executed by a CPU 5b appearing in FIG. 1;

FIG. 3 is a subroutine carried out at a step 122 appearing in FIG. 2;

FIG. 4 shows a T.sub.OUT - V.sub.HCFLVL table used at a step 203 appearing in FIG. 3:

FIG. 5 is a flowchart of a subroutine carried out at a step 125 appearing in FIG. 2; and

FIG. 6 shows a T.sub.OUT - V.sub.HCRLVL table used at a step 301 appearing in FIG. 5.

The invention will now be described in detail with reference to the drawings showing an embodiment thereof.

Referring first to FIG. 1, there is shown the whole arrangement of a fuel supply control system for an internal combustion engine, including a system for detecting deterioration of HC sensors according to the invention. In the figure, reference numeral 1 designates an internal combustion engine for automotive vehicles. Connected to the cylinder block of the engine 1 is an intake pipe 2 across which is arranged a throttle body 3 accommodating a throttle valve 3' therein. A throttle valve opening (.theta..sub.TH) sensor 4 is connected to the throttle valve 3' for generating an electric signal indicative of the sensed throttle valve opening and supplying same to an electronic control unit (hereinafter called "the ECU") 5.

Fuel injection valves 6, only one of which is shown, are inserted into the interior of the intake pipe 2 at locations intermediate between the cylinder block of the engine I and the throttle valve 3' and slightly upstream of respective intake valves, not shown. The fuel injection valves 6 are connected to a fuel pump, not shown, and electrically connected to the ECU 5 to have their valve opening periods controlled by signals therefrom.

On the other hand, an intake pipe absolute pressure (P.sub.BA) sensor 8 is provided in communication with the interior of the intake pipe 2 through a conduit 7 at a location immediately downstream of the throttle valve 3' for supplying an electric signal indicative of the sensed absolute pressure within the intake pipe 2 to the ECU 5.

An engine coolant temperature (T.sub.W) sensor 9, which may be formed of a thermistor or the like, is mounted in the cylinder block of the engine 1, for supplying an electric signal indicative of the sensed engine coolant temperature T.sub.W to the ECU 5. An engine rotational speed (Ne) sensor 10 and a cylinder-discriminating (CYL) sensor II are arranged in facing relation to a camshaft or a crankshaft of the engine 1, neither of which is shown. The engine rotational speed sensor 10 generates a pulse as a TDC signal pulse at each of predetermined crank angles whenever the crankshaft rotates through 180 degrees, while the cylinder-discriminating sensor 11 generates a pulse at a predetermined crank angle of a particular cylinder of the engine, both of the pulses being supplied to the ECU 5. The ECU 5 calculates an engine rotational speed Ne based on the TDC signal pulses.

A three-way catalyst (CAT) 13 is arranged within an exhaust pipe 12 connected to the cylinder block of the engine 1 for purifying noxious components such as HC, CO, and NOx. An O.sub.2 sensor 14 as an oxygen concentration sensor is mounted in the exhaust pipe 12 at a location intermediate between the three-way catalyst 13 and the engine 1, for sensing the concentration of oxygen present in exhaust gases emitted therefrom and supplying an electric signal in accordance with an output value thereof to the ECU 5. Further, a catalyst temperature (T.sub.CAT) sensor 15 is mounted on the three-way catalyst 13 for detecting the temperature of same and supplying a signal indicative of the detected catalyst temperature T.sub.CAT to the ECU 5.

Further, HC sensors 16, 17 are arranged in the exhaust pipe 12 at locations upstream and downstream of the three-way catalyst 13, respectively, for detecting the concentration of hydrocarbons (HC) present in exhaust gases, and supplying signals having output voltages corresponding to the detected concentration of hydrocarbons to the ECU 5. The HC sensors 16, 17 each have a characteristic that as the concentration of hydrocarbons in exhaust gases increases, its output voltage increases.

The ECU 5 detects deterioration of the three-way catalyst 13 by comparing between signals supplied from the HC sensor (hereinafter referred to as "the pre-catalyst catalyst HC sensor") 16 upstream of the three-way catalyst 13 and the HC sensor (hereinafter referred to as "the post-catalyst HC sensor") 17 downstream of same, respectively. The manner of detection of deterioration of the three-way catalyst 13 is disclosed in U.S. Ser. No. 07 717,247, referred to hereinbefore.

Connected to the ECU 5 is an indicator 18 formed of four LED s (light emitting diodes) for raising an alarm when abnormality of the HC sensors 16, 17 has been detected in a manner described in detail hereinafter.

The ECU 6 comprises an input circuit 5a having the functions of shaping the waveforms of input signals from various sensors, shifting the voltage levels of sensor output signals to a predetermined level, converting analog signals from analog-output sensors to digital signals, and so forth, a central processing unit (hereinafter called "the CPU") 5b for executing a performance monitoring program including steps for deterioration detection described hereinafter etc., memory means 5c storing various operational programs which are executed in the CPU 5b, and a Ti map, a T.sub.OUT - V.sub.HCFLVL table, and a T.sub.OUT - V.sub.HCRLVL table, described hereinafter, and for storing results of calculations therefrom, etc., and an output circuit 5d which outputs driving signals to the fuel injection valves 6, the indicator 18, etc.

In addition, the ECU 5 forms difference-calculating means, comparison means, judging means, memory means, and correcting means, recited in the appended claims.

The CPU 5b operates in response to output signals from various sensors to determine operating conditions in which the engine 1 is operating, such as an air-fuel ratio feedback control region in which the fuel supply is controlled in response to the detected oxygen concentration in the exhaust gases, and open-loop control regions including a fuel cut region, and calculates, based upon the determined operating conditions, the valve opening period or fuel injection period T.sub.OUT over which the fuel injection valves 6 are to be opened, by the use of the following equation (1) in synchronism with inputting of TDC signal pulses to the ECU 5:

where T.sub.i represents a basic value of the fuel injection period T.sub.OUT of the fuel injection valves 6, which is read from a Ti map set in accordance with the engine rotational speed Ne and the intake pipe absolute pressure P.sub.BA.

K.sub.O2 is an air-fuel ratio feedback control correction coefficient whose value is determined in response to the oxygen concentration in the exhaust gases detected by the O.sub.2 sensor 14, during feedback control, while it is set to respective predetermined appropriate values while the engine is in predetermined operating regions (the open-loop control regions) other than the feedback control region.

The correction coefficient KO.sub.2 is calculated by known proportional control using a proportional term (P-term) when an output level V.sub.O2 of the O.sub.2 sensor 14 is inverted with respect to a reference value, and by known integral control using an integral term (I-term) when the former is not inverted with respect to the latter (the manner of this calculation is disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 63-189638).

K.sub.1 and K.sub.2 are other correction coefficients and correction variables, respectively, which are calculated based on various engine parameter signals to such values as to optimize characteristics of the engine such as fuel consumption and driveability depending on operating conditions of the engine.

The CPU 5b supplies through the output circuit 5d, the fuel injection valves 6 with driving signals corresponding to the calculated fuel injection period T.sub.OUT determined as above, over which the fuel injection valves 6 are opened.

The manner of monitoring the performance of the HC sensors 16, 17 including detection of deterioration of same, which is carried out by the CPU 5b, will now be described in detail with reference to FIGS. 2a, 2b and 2c showing a control program therefor. The control program is executed whenever a TDC signal pulse is inputted to the ECU 5.

First, at a step 101, it is determined whether or not the engine 1 is in a starting mode. If the answer to this question is affirmative (Yes), a t.sub.HCCHKDLY timer formed of a down counter for measuring time elapsed after the engine 1 left the starting mode is set to a predetermined time period t.sub.HCCHKDLY (e g. 60 seconds) required to elapse until the HC sensors 16, 17 are activated after being heated, and started at a step 102. Further, at a step 103, a zero point correction value V.sub.HCFL for the pre-catalyst HC sensor 16 and a zero point correction value V.sub.HCRL for the post-catalyst HC sensor 17 are initialized by setting both of them to 0. Then, at a step 104, as an initial value of a learned average value V.sub.HCFCHKAV of output from the pre-catalyst HC sensor, a present value V.sub.HCFAD (A/D converted value) of output from the HC sensor 16 is set, and at a step 105, a t.sub.HCFLCHK timer formed of a down counter for measuring duration of abnormality in the zero point of output from the pre-catalyst HC sensor 16 is set to a predetermined time period t.sub.HCFLCHK (e.g. 5 seconds) and started, and a t.sub.HCRLCHK timer formed of a down counter for measuring duration of abnormality in the zero point of output from the post-catalyst HC sensor 17 is set to a predetermined time period t.sub.HCRLCHK (e g. 5 seconds) and started. At the following step 106, a t.sub.HCFHCHK timer formed of a down counter for measuring duration of non-zero point abnormality of the pre-catalyst HC sensor 16 (abnormality in an output range other than the zero point) is set to a predetermined time period t.sub.HCFHCHK (e.g. 5 seconds) and started, and a t.sub.HCRHCHK timer formed of a down counter for measuring duration of non-zero point abnormality of the post-catalyst HC sensor 17 is set to a predetermined time period t.sub.HCRHCHK (e.g. 5 seconds) and started, followed by terminating the present program.

On the other hand, if the answer to the question of the step 101 is negative (No), it is determined at a step 107 whether or not the count value of the t.sub.HCCHKDLY timer is equal to 0. If the answer to this question is negative (No), the program proceeds to the step 103, whereas if the answer is affirmative (Yes), i.e. if the predetermined time period t.sub.HCCHKDLY has elapsed after the engine 1 left the starting mode, the program proceeds to a step 108.

At the step 108, it is determined whether or not a flag F.sub.--CRS for indicating the state of cruising of a vehicle on which the engine 1 is installed is equal to 1. The flag F.sub.--CRS is set to 1 in another routine when a change in the travelling speed of the vehicle in two seconds is smaller e.g. than 0.8 km/h. The answer to the question of the step 108 is initially negative (No), so that the program proceeds to a step 109.

At the step 109, it is determined whether or not fuel cut (inhibition of fuel supply to the engine) is being carried out in the present loop. Further, at a step 110, it is determined whether or not the fuel cut was carried out in the immediately preceding loop. If either of the answers to the questions of the steps 109 and 110 is negative (No), the program proceeds to the step 104, whereas both the answers are affirmative (Yes), i.e the fuel out was carried out in the immediately preceding loop and is being carried out in the present loop, the program proceeds to steps 111 to 120 to set the zero point correction values V.sub.HCFL, V.sub.HCRL for the HC sensors 16, 17 and detect whether or not there is abnormality in the zero points of output from same.

Specifically, at a step 111, it is determined whether or not a present value V.sub.HCFAD of output from the pre-catalyst HC sensor 16 is larger than an upper limit value V.sub.HCLLMT (e.g. 50 mV) of zero point deviation. If the answer to this question is negative (No), it is judged that there is no zero point abnormality in the pre-catalyst HC sensor 16, i.e. there is no abnormality such that when the engine undergoes fuel cut, during which the output from a normally functioning HC sensor should assume a value of 0, the HC sensor outputs voltage higher than a predetermined value, and the zero point correction value V.sub.HCFL for the pre-catalyst HC sensor 16 is set to the present value V.sub.HCFAD of output therefrom and stored in the memory means at a step 112. Then, at a step 113, the t.sub.HCFLCHK timer is set to the predetermined time period t.sub.HCFLCHK and started, followed by the program proceeding to a step 116. The zero point correction value V.sub.HCFL thus set is used for correcting the output value from the pre-catalyst HC sensor 16 in a step 203 appearing in FIG. 3, referred to hereinafter.

On the other hand, if the answer to the question of the step 111 is affirmative (Yes), it is provisionally judged that there is zero point abnormality occurring in the pre-catalyst HC sensor 16, and then it is determined at a step 114 whether or not the count value of the t.sub.HCFLCHK timer is equal to 0. If the answer to this question is negative (No), the program proceeds to the step 116, whereas if the answer is affirmative (Yes), i.e. if the present value V.sub.HCFAD of output from the pre-catalyst HC sensor 16 has continued to be larger than the upper limit value V.sub.HCLLMT over the predetermined time period t.sub.HCFLCHK, it is finally judged that there is zero point abnormality occurring in the pre-catalyst HC sensor 16, and then a flag F.sub.13 HCFLVNG for indicating zero point abnormality of the sensor 16 is set to 1 at a step 115, followed by the program proceeding to the step 116.

At the step 116, it is determined whether or not a present value (A/D converted value V.sub.HCRAD of output from the post-catalyst HC sensor 17 is larger than the upper limit value V.sub.HCLLMT of zero point deviation. If the answer to this question is negative (No), it is judged that there is no zero point abnormality occurring in the post-catalyst HC sensor 17, and the zero point correction value V.sub.HCRL for the post-catalyst catalyst HC sensor 17 is set to the present value V.sub.HCRAD of output therefrom and stored in the memory means at a step 117. Then the t.sub.HCRLCHK timer is set to the predetermined time period t.sub.HCRLCHK and started at a step 118, followed by the program proceeding to the step 106. The zero point correction value V.sub.HCRL is used for correcting the output value from the post-catalyst HC sensor 17 at a step 301 appearing in FIG. 5, referred to hereinafter.

If the answer to the question of the step 116 is affirmative (Yes), it is provisionally judged that there is possibility of occurrence of zero point abnormality in the post-catalyst HC sensor 17, and then it is determined at a step 119 whether or not the count value of the t.sub.HCRLCHK is equal to 0. If the answer to this question is negative (No), the program proceeds to the step 106, whereas if the answer is affirmative (Yes), i.e. if the present value V.sub.HCRAD of output from the post-catalyst HC sensor 17 has continued to be larger than the upper limit value V.sub.HCLLMT of zero point deviation over the predetermined time period t.sub.HCRLCHK, it is finally judged that there is zero point abnormality occurring in the post-catalyst HC sensor 17, and then a flag F.sub.--HCRLVNG for indicating the zero point abnormality of the post-catalyst HC sensor 17 is set to 1 at a step 120, followed by the program proceeding to the step 106.

When the vehicle starts cruising and the answer to the question of the step 108 becomes affirmative (Yes), the program proceeds to a step 121, where it is determined whether or not the air-fuel ratio feedback control based on output from the O.sub.2 sensor 14 is being carried out. If the answer to this question is affirmative (Yes), i.e. if the vehicle is cruising and at the same time the air-fuel ratio feedback control is being carried out, it is judged that the engine is in a condition suitable for detecting non-zero point abnormality in the pre-catalyst HC sensor 16, so that the program proceeds to a step 122 to detect non-zero point abnormality in the pre-catalyst HC sensor 16. The non-zero point abnormality is abnormality in the output value of the HC sensor assumed when fuel is being supplied to the engine 1 and hence hydrocarbons are being emitted into exhaust gases. On the other hand, if the answer to the question of the step 121 is negative (No), it is judged that engine is not in a condition suitable for detecting non-zero point abnormality, and the t.sub.HCFHCHK timer is set to the predetermined time period t.sub.HCFHCHK and started at a step 123, followed by the program proceeding to a step 124.

Details of the step 122 are shown in FIG. 3 showing a subroutine SUB1 for detection of non-zero point abnormality of the pre-catalyst HC sensor 16.

First, at a step 201, it is determined whether or not the output level V.sub.O2 of the O.sub.2 sensor 14 has been inverted with respect to the reference value. If the answer to this question is affirmative (Yes), the learned average value V.sub.HCFCHKAV of output values V.sub.HCFRAD from the pre-catalyst HC sensor 16 is calculated at a step 202 by the following equation (2):

where V.sub.HCFCHKAV on the right-hand side is a value of the learned average value obtained up to the immediately preceding loop, using the value set at the step 104 in FIG. 2 as its initial value, and C.sub.HCCHK is a value selected from a value range of 1 to 100.

If the answer to the step 201 is negative (No), the program skips over the step 202 to a step 203.

At the step 203, a deviation V.sub.HCFDEL in the output from the pre-catalyst HC sensor 16 is calculated by the following equation (3) using the learned average value V.sub.HCFCHKAV obtained up to the present loop:

where V.sub.HCFL is the zero point correction value ser at the step 112 in FIG. 2, and as can be learned from this equation, the learned average value V.sub.HCFCHKAV is subjected to zero point correction by subtracting the value V.sub.HCFL therefrom.

V.sub.HCFLVL is a standard value (predetermined reference value) of output from the pre-catalyst HC sensor 16 which is set in accordance with the fuel injection period T in a T.sub.OUT - V.sub.HCFLVL table shown in FIG. 4. The T.sub.OUT - V.sub.HCFLVL table is set based on the fact that the concentration of hydrocarbons in exhaust gases emitted during the air-fuel ratio feedback control is commensurate to an amount of fuel supplied to the engine, and therefore it is possible to predict a standard value of output from an HC sensor from the fuel injection period T.sub.OUT, which corresponds to the amount of fuel supplied to the engine.

Then, at a step 204, it is determined whether or not the deviation V.sub.HCFDEL in the output from the pre-catalyst HC sensor 16 obtained at the step 203 is larger than an upper limit value V.sub.HCDELLMT (e.g. 20 mV). If the answer to this question is negative (No), the t.sub.HCFHCHK timer is set to the predetermined time period t.sub.HCFHCHK, and started at a step 205, followed by the program proceeding to the step 124 in FIG. 2. On the other hand, if the answer to the question of the step 204 is affirmative (Yes), it is provisionally judged that there is non-zero point abnormality occurring in the pre-catalyst HC sensor 16, and it is determined at a step 206 whether or not the count value of the t.sub.HCFHCHK timer is equal to 0.

If the answer to the question of the step 206 is negative (No), the program immediately proceeds to the step 124 in FIG. 2, whereas if the answer is affirmative (Yes), i.e. if the deviation V.sub.HCFDEL in the output from the pre-catalyst HC sensor 16 has continued to be larger than the upper limit value V.sub.HCDELLMT over the predetermined time period t.sub.HCFHCHK, it is finally judged that there is non-zero point abnormality occurring in the pre-catalyst HC sensor 16, and then a flag F.sub.--HCFLVLNG for indicating the non-zero point abnormality of the sensor 16 is set to 1 at a step 207, followed by the program proceeding to the step 124 in FIG. 2.

Referring again to FIG. 2, at the step 124, it is determined whether or not the catalyst temperature T.sub.CAT is lower than a predetermined value T.sub.HCRLVLCHK (e.g. 200.degree. C.). The predetermined value T.sub.HCRLVLCHK is set at a lower limit value of a catalyst temperature range within which the three-way catalyst can exhibit normal purifying efficiency if it is normally functioning. Therefore, the step 124 is provided for determining whether or not the three-way catalyst has lost its normal purifying ability and hence hydrocarbons of high concentration are supplied to the post-catalyst HC sensor 17.

If the answer to the question of the step 124 is affirmative (Yes), i.e. if the vehicle is cruising and at the same time the catalyst temperature T.sub.CAT is lower than the predetermined value T.sub.HCRLVLCHK, it is judged that the engine is in a condition suitable for detecting non-zero point abnormality in the post-catalyst HC sensor 17, and the program proceeds to a step 125 to detect non-zero point abnormality in the post-catalyst HC sensor 17. On the other hand, if the answer to the question of the step 124 is negative (No), it is judged that the engine is not in a condition suitable for detecting non-zero point abnormality, and the t.sub.HCRHCHK timer is set to the predetermined time period t.sub.HCRHCHK and started at a step 126, followed by the program proceeding to a step 127.

Details of the step 125 are shown in FIG. 5 showing a subroutine SUB 2 for detection of non-zero point abnormality of the post-catalyst HC sensor 17.

First, at a step 301, a deviation V.sub.HCRDEL in the output from the post-catalyst HC sensor 17 is calculated by the following equation (4) using the present value V.sub.HCRAD of output from the post-catalyst HC sensor 17:

where V.sub.HCRL is the zero point correction value set at the step 117 in FIG. 2, and as can be learned from this equation, the present value V.sub.HCRAD is subjected to zero point correction by subtracting the value V.sub.HCRL therefrom.

V.sub.HCRLVL is a standard value (predetermined reference value) of output from the post-catalyst HC sensor 17 which is set in accordance with the fuel injection period T.sub.OUT and the catalyst temperature T.sub.CAT in a T.sub.OUT - V.sub.HCRLVL table shown in FIG. 6. The standard value V.sub.HCRLVL is set such that it increases with an increase in the fuel injection period T.sub.OUT, and it decreases with an increase in the catalyst temperature T.sub.CAT insofar as the T.sub.OUT value is the same. When the catalyst temperature T.sub.CAT lies between a value T.sub.CAT1 and a value T.sub.CAT2 (>T.sub.CAT1), the standard value V.sub.HCRLVL is calculated by interpolation.

As noted above, in detection of non-zero point abnormality in the post-catalyst HC sensor 17, the calculation of a learned average value of output from the sensor is not carried out as in the case of detection of non-zero point abnormality in the pre-catalyst HC sensor 16 (the step 202 in FIG. 3). This is because the concentration of HC has already been averaged by the three-way catalyst 13, and this makes unnecessary the use of the learned average value in the case of the post-catalyst HC sensor 17 arranged downstream of the catalyst 13. However, the learned average value may be calculated with respect to the post-catalyst HC sensor 17 as well to obtain the output deviation V.sub.HCRDEL thereof.

Then, at a step 302, it is determined whether or not the output deviation V.sub.HCRDEL of the post-catalyst HC sensor 17 obtained at the step 301 is larger than the upper limit value V.sub.HCDELLMT. If the answer to this question is negative (No), the t.sub.HCRHCHK timer is set to the predetermined time period t.sub.HCRHCHK, and started at a step 303, followed by the program proceeding to the step 127 in FIG. 2. On the other hand, if the answer to the question of the step 302 is affirmative (Yes), it is provisionally judged that there is non-zero point abnormality in the post-catalyst HC sensor 17, and it is determined at a step 304 whether or not the count value of the t.sub.HCRHCHK timer is equal to 0.

If the answer to the question of the step 304 is negative (No), the program immediately proceeds to the step 127 in FIG. 2, whereas if the answer is affirmative (Yes), i.e. if the deviation V.sub.HCRDEL in output from the post-catalyst HC sensor has continued to be larger than the upper limit value V.sub.HCDELLMT over the predetermined time period t.sub.HCRHCHK, it is finally judged that there is non-zero point abnormality occurring in the post-catalyst HC sensor 17, and then a flag F.sub.--HCRLVLNG for indicating the non-zero point abnormality of the sensor 17 is set to 1 at a step 305, followed by the program proceeding to the step 127 in FIG. 2.

Referring again to FIG. 2, at the step 127, the t.sub.HCFLCHK timer and the t.sub.HCRLCHK timer are set to the respective predetermined time periods t.sub.HCFLCHK and t.sub.HCRLCHK, and started, respectively, followed by terminating the present program.

In another control program, not shown, it is determined whether or not the flags F.sub.--HCFLVNG and F.sub.--HCFLVLNG for respectively indicating the zero point abnormality and the non-zero point abnormality in the pre-catalyst HC sensor 16, and the flags F.sub.--HCRLVNG and F.sub.--HCRLVLNG for respectively indicating the zero point abnormality and the non-zero point abnormality in the post-catalyst HC sensor 17 are each equal to 1. If any of the flags assumes a value of 1, a driving signal is supplied to the indicator 18 such that an LED corresponding to the flag assuming a value of 1 is lighted. Thus, the driver or a car mechanic can be informed of abnormality of the HC sensors.

Although in the above described embodiment, the system for detecting deterioration of HC sensors is applied to an internal combustion engine having two HC sensors, this is not limitative but it goes without saying that the system may be applied to any engine having at least one HC sensor mounted therein.

Further, although in the above described embodiment, the HC sensors are used to detect deterioration of the three-way catalyst, it goes without saying that the method according to the invention may be applied to various cases in which one or more HC sensors are used for other applications such as air-fuel ratio control, fuel supply control and intake air amount control control, which are carried out in response to output from the HC sensor or sensors.