DTC P0420: Catalyst System Efficiency Below Threshold (Bank 1)

OUTLINE OF DIAGNOSIS
Detect the deterioration of catalyst function.

Though the rear oxygen sensor output would change slowly with a new catalyst, the sensor output with a deteriorated catalyst becomes high and the inversion time is shortened. For this reason, the catalyst diagnosis is carried out by monitoring the rear oxygen sensor output and comparing it with the front A/F sensor output.
COMPONENT DESCRIPTION

Fig 1: DTC P0420 - Catalyst System Efficiency Below Threshold (Bank 1) - Component Description
Courtesy of SUBARU OF AMERICA, INC.

ENABLE CONDITION

ENABLE CONDITION CHART

Secondary Parameters	Enable Conditions
Time for keep completing all secondary parameters	1.2 seconds or more
Battery voltage	> 10.9 V
Atmospheric pressure	> 75.1 kPa (563 mmHg, 22.2 inHg)
Engine coolant temperature	"GREATER THAN EQUAL TO" 5°C (149°F)
Catalyst warm-up counter on Map 2	"GREATER THAN EQUAL TO" 7600
Misfire detection during 200 engine revs.	< 5 times
Learning value of evaporation gas density	<0.20
Sub feedback	Operating
Evaporative system diagnostic	Not in operation
Difference between actual and target time lambda < 0.10	1000 milliseconds or more
Vehicle speed	"GREATER THAN EQUAL TO" 75 km/h (46.6 MPH)
Amount of intake air	10 <-->40 g/s
Engine load change every 0.5 engine revs.	< 0.03 g/rev
Rear O₂ output change from below 600 mV to over	Experienced after fuel cut
After engine starting	"GREATER THAN EQUAL TO" 200 seconds
Engine speed	1600 - 3500 rpm

Map 2

Add the following value every 512 milliseconds.

Catalyst warm-up counter "LESS THAN EQUAL TO" 9000

VALUE ADD CHART

Amount of intake air (g/s)	0	5	7.5	10	15	20	25	30	35	40	45	50
Integrated value for warm-up counter	-18	-5	3	11	25	40	57	72	87	100	100	100

Catalyst warm-up counter > 9000

CATALYST WARM-UP COUNTER CHART

Amount of intake air (g/s)	0	5	7.5	10	15	20	25	30	35	40	45	50
Integrated value for warm-up counter	-8	-4	-1	3	6	8	9	11	13	14	14	14

GENERAL DRIVING CYCLE
Engine is warmed-up, then perform the diagnosis only once at a constant more than 75 km/h (46.6 MPH).

DIAGNOSTIC METHOD

After the malfunction criteria are completed, calculate the output fluctuation value of front oxygen (A/F) sensor and output fluctuation value of rear oxygen sensor.

Calculate the diagnosis value when the front oxygen (A/F) sensor output fluctuation value more than specified value.

Regard the A/F response properties and diagnosis value as parameters for judgment value.

Judge NG when the malfunction criteria below are completed, and judge OK when they are not completed.

Judgment Value

JUDGMENT VALUE CHART

Malfunction Criteria	Threshold Value
Accumulated variation of output voltage of rear oxygen sensor per 32 milliseconds divided by accumulated variation of lambda of front oxygen (A/F) sensor per 32 milliseconds	"GREATER THAN EQUAL TO" .5

Time Needed for Diagnosis: 55 seconds

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

DTC CLEAR CONDITION
- When the OK idling cycle was completed 40 times in a row
- When "Clear Memory" was performed
MALFUNCTION INDICATOR LIGHT CLEAR CONDITION
- When the OK driving cycle was completed 3 times in a row
- When "Clear Memory" was performed
FAIL SAFE
None
ECM OPERATION AT DTC SETTING
- Memorize the freeze frame data. (For test mode $02)
- Memorize the diagnostic value and trouble standard value. (For test mode $06)