Outline Of Diagnosis

Detect the slow response of front oxygen (A/F) sensor.

Front oxygen (A/F) sensor cover has some ventilation holes for exhaust gas. Clogged ventilation holes are diagnosed.

When the holes are clogged, the A/F output variation becomes slow comparing with the actual A/F variation because oxygen which reaches the zirconia layer is insufficient. Therefore, if the cover has clogged holes, the rich to lean judgment in ECM is delayed when the change from rich to lean is caused.

Fig 1: Identifying Front Oxygen (A/F) Sensor

Courtesy of SUBARU OF AMERICA, INC.

When abnormal, the variation period from rich to lean is longer than when normal, as shown in the figure below.

When the condition is completed, the variation time from rich to lean and vice versa is calculated by varying the desired A/F value. Judge NG when the period is long.

Fig 2: Variation Period Graph

Courtesy of SUBARU OF AMERICA, INC.