Subaru 2009 IMPREZA Service Manual page 821

Diagnostic Trouble Code (DTC) Detecting Criteria
BE: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
(B) Deterioration
(1) Front oxygen (A/F) sensor
3. ENABLE CONDITIONS
Secondary Parameters
Battery voltage
Barometric pressure
Engine coolant temperature
Estimated catalyst temperature
Misfire detection every 200 rotations
Learning value of evaporation gas density
Sub feedback
Evaporative system diagnosis
Time of difference (< 0.10) between actual
lambda and target lambda
Vehicle speed
Amount of intake air
Engine load change every 0.5 engine revs.
Rear oxygen output change from 660 mV or
less to 660 mV or more
Elapsed time after starting the engine
Purge execution calculated time
(1)
(C)
(C) Output waveform from the front
oxygen (A/F) sensor
(2) Front oxygen sensor
Enable Condition
10.9 V
75 kPa (563 mmHg, 22.2 inHg)
70 °C (158 °F)
540 °C (1004 °F)
< 5 time(s)
< 0.2
In operation
Not in operation
1000 ms
> 70 km/h (43.5 MPH)
10 g/s (0.35 oz/s)
and
< 40 g/s (1.41 oz/s)
< 0.02 g/rev (0 oz/rev)
Experienced after fuel cut
205 s
0 s
GD(H4SO)-97
(2)
(3)
(3)
(D)
(3)
GENERAL DESCRIPTION
(D)
EN-01713
Output waveform from the rear
oxygen Sensor
Catalytic converter