Diagnostic Method

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

When the oxygen sensor output voltage changes from 0.55 V (rich) to 0.15 V (lean), calculate the minimum response time for output change between 0.5 V and 0.2 V for the judgment criteria.

Fig 1: Rear Oxygen Sensor Voltage Graph

Courtesy of SUBARU OF AMERICA, INC.

Abnormality Judgment

Judge as NG when the judgment value is larger than the threshold value after deceleration fuel cut. Response time (diagnosis value) > threshold value → abnormal
NOTE: Variation time of rear oxygen sensor output voltage is short during fuel shut-off in deceleration. Carry out the NG judgment only after the fuel shut-off in deceleration. Even without deceleration fuel cut, judge as OK if the value is below the threshold.

When the deceleration fuel cut time is more than 6 seconds, judge as NG if the following criteria are met 2 seconds after recovering from the deceleration fuel cut.
Judge as NG when the oxygen sensor voltage at recovery from a deceleration fuel cut is large.
If the fuel cut time in a deceleration fuel cut is long (more than 6 s), and even after recovering from a deceleration fuel cut, the oxygen sensor voltage is high (0.55 V or more), judge as NG.

Judgment Value

JUDGMENT VALUE CHART

Malfunction Criteria	Threshold Value
Shortest time change from rich (0.5 V O₂ output) to lean (0.2 V) when voltage reduces from 0.55 V to 0.15 V	> 0.837 seconds
Time at 0.55 V or more	> 2 seconds

Time Needed for Diagnosis: 1 time

Malfunction Indicator Light Illumination: Illuminates when malfunction occurs in 2 continuous driving cycles.

Normality Judgment

Regardless of a deceleration fuel cut, if the response time (diagnostic value) when the oxygen sensor voltage has changed from rich to lean is shorter than the threshold value (judgment value), judge as a normal condition.
Response time (diagnostic value) ≤ threshold value → normal
Normality judgement is not to be performed.
Judge as OK when the criteria below are met.

Judgment Value

JUDGMENT VALUE CHART

Malfunction Criteria	Threshold Value
Shortest time change from rich (0.5 V O₂ output) to lean (0.2 V) when voltage reduces from 0.55 V to 0.15 V	≤ 0.837 second

Calculate the minimum value of 0.3 V to 0.5 V output change response time as judgment value, when the oxygen sensor output voltage changes from 0.25 V (lean) to 0.55 V (rich).

Fig 1: Rear O2 Sensor Voltage Graph

Courtesy of SUBARU OF AMERICA, INC.

Abnormality Judgment

Judge as NG when the judgment value is larger than the threshold value after deceleration fuel cut. Response time (diagnosis value) > threshold value → abnormal
If the oxygen sensor voltage is small after recovering from a deceleration fuel cut, and remains small, judge as NG.

Judgment Value

JUDGMENT VALUE CHART

Malfunction Criteria	Threshold Value
Shortest time change from lean (0.3 V O₂ output) to rich (0.5 V) when voltage reduces from 0.5 V to 0.25 V	> 2 seconds
Time at less than 0.25 V	> 120 seconds

Time Needed for Diagnosis: 1 time

Malfunction indicator Light Illumination: Illuminates when malfunction occurs in 2 continuous driving cycles.

Normality Judgment

Regardless of a deceleration fuel cut, if the response time (diagnostic value) when the oxygen sensor voltage has changed from rich to lean is shorter than the threshold value (judgment value), judge as a normal condition.
Response time (diagnostic value) ≤ threshold value → normal
Do not judge as a normal condition.
Judge as OK when the criteria below are met.

Judgment Value

JUDGMENT VALUE CHART

Malfunction Criteria	Threshold Value
Shortest time change from lean (0.3 V O₂ output) to rich (0.5 V) when voltage reduces from 0.55 V to 0.25 V	≤ 2 sec.