Subaru 2009 IMPREZA Service Manual page 1699

Diagnostic Trouble Code (DTC) Detecting Criteria
BJ:DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)
1. OUTLINE OF DIAGNOSIS
Detect the deterioration of the catalyst function.
Though the front oxygen sensor output would change slowly with a new catalyst, the sensor output with a de-
teriorated catalyst becomes high and the inversion time is shortened.
For this reason, the catalyst diagnosis is carried out by monitoring the front oxygen sensor output and com-
paring it with the front oxygen (A/F) sensor output.
2. COMPONENT DESCRIPTION
(A)
(B)
(A) Normal
(D) Output waveform from the rear
oxygen Sensor
(1) Front oxygen (A/F) sensor
(1)
(C)
(B) Deterioration
(2) Front oxygen sensor
GD(H4DOTC)-111
(2)
(3)
(3)
(C)
(3)
GENERAL DESCRIPTION
(D)
EN-01713
Output waveform from the front
oxygen (A/F) sensor
Catalytic converter