Outline Of Diagnosis

Detect the slow response of oxygen sensor.

Rich → Lean response diagnosis, Lean → Rich response diagnosis:

Judge NG, if in this case of either NG.

Judge OK, in this case of both OK.

[Rich → Lean response diagnosis]

1. Measure the response time of oxygen sensor output signal change when A/F ratio is changed from rich to lean. Judge NG when response time is larger as from threshold value. Judge OK when it time is smaller as from threshold value.
2. Judge NG when oxygen sensor voltage after the recovery of fuel cut at deceleration is high (rich).

[Lean → Rich response diagnosis]

1. Measure the response time of oxygen sensor output signal change when A/F ratio is changed from lean to rich. Judge NG when response time is larger as from threshold value.
2. Judge NG when oxygen sensor voltage after the recovery of fuel cut at deceleration is low and oxygen sensor voltage remains at low voltage.

• Diagnostic Method 

Measure the response time of the output change of the oxygen sensor when the A/F ratio changes from rich to lean. And Judge NG when the measured response time is larger than the threshold value.