Deterioration diagnosis system for an air-fuel ratio sensor

1. A deterioration diagnosis system of an air fuel ratio sensor mounted to an exhaust pipe of an internal combustion engine for determining a mixture ratio of air and fuel fed into the internal combustion engine, comprising:

a means for calculating an inclination of an output signal of the air-fuel ratio sensor,

a means for calculating variance of the inclination, and

a means for determining deterioration of the air-fuel ratio sensor based on

the variance of the inclination

wherein the inclination variance of the output signal of the air-fuel ratio sensor is corrected in accordance with the internal combustion engine operation condition, the corrected variances obtained at predetermined times are summed, and the air-fuel ratio sensor deterioration is determined based on the calculated sum.

2. A deterioration diagnosis system of an air fuel ratio sensor according to claim 1 , wherein the output signal from the air-fuel ratio sensor is replaced by an instruction value to a fuel injection system.

3. A deterioration diagnosis system for an air fuel sensor mounted to an exhaust pipe of an internal combustion engine for determining a mixture ratio of air and fuel fed into the internal combustion engine, comprising:

a means for calculating an inclination of an output signal of. the air-fuel ratio sensor,

a means for calculating variance of the inclination,

a means for calculating variance of the output signal of the air-fuel ratio sensor,

a means for normalizing the variance of the inclination by using the variance of the output signal of the air-fuel ratio sensor; and a means for determining deterioration of the air-fuel ratio sensor based on the normalized variance of the inclination.

4. A deterioration diagnosis system of an air fuel ratio sensor according to claim 3 ,

wherein the normalized inclination variance of the output signal of the air-fuel ratio sensor is corrected in accordance with the internal combustion engine operation condition, the corrected normalized variances obtained at predetermined times are summed, and the air-fuel ratio sensor deterioration is determined based on the calculated sum.

5. A deterioration diagnosis system of an air fuel ratio sensor according to claim 3 , wherein the output signal from the air-fuel ratio sensor is replaced by an instruction value to a fuel injection system.

6. A deterioration diagnosis system of an air fuel sensor according to claim 3 , wherein if deterioration is determined, the system informs an operator of the deterioration by turning on an indicator lamp.

7. A deterioration diagnosis system for an air fuel sensor mounted to an exhaust pipe of an internal combustion engine for determining a mixture ratio of air and fuel fed into the internal combustion engine, comprising:

a means for calculating two inclinations of an output signal of an air-fuel ratio sensor by obtaining differences of the output signal in different time intervals;

a means for calculating variance of each inclination; and

a means for calculating a ratio of these variances of the output signals of the air-fuel ratio sensor, thereby determining deterioration of the air-fuel ratio sensor based on the ratio of the variances of the output signals.

8. A deterioration diagnosis system of an air fuel sensor according to claim 7 ,

wherein a ratio of two inclination variances of the output signals of the air-fuel ratio sensor is corrected in accordance with the engine operation condition, the corrected ratios obtained at predetermined times are summed, and the air-fuel ratio sensor deterioration is determined based on the calculated sum.