DTC P0420: Catalyst System Efficiency Below Threshold (Bank1)

OUTLINE OF DIAGNOSIS
Check the catalyst efficiency deterioration.

Generally, the catalytic converter exhaust gas conversion efficiency and oxygen absorbing/removing function are inter-related. The front A/F sensor and rear oxygen sensor are therefore utilized to monitor the oxygen absorbing/removing function, and the results are used to monitor the conversion efficiency.

Fig 1: Checking Catalyst Efficiency Deterioration
Courtesy of SUBARU OF AMERICA, INC.

Oil adherence to the catalytic metal (an alloy of platinum and rhodium, or an alloy of platinum, rhodium and palladium) in catalyst converter or metal melting by extraordinary high temperature result in the decrease of metal surface area contacting exhaust gas. Eventually, catalytic conversion efficiency blunted and oxygen density of catalyst downstream may change.

Fig 2: Identifying Metal (Catalytic Function) And Monolith
Courtesy of SUBARU OF AMERICA, INC.

Output of rear oxygen sensor changes gradually with a new catalyst, but the blunted catalyst efficiency mentioned above makes sensor output larger and inverse time shorter.

Therefore, catalyst diagnosis is conducted by monitoring the rear oxygen sensor output and comparing its output with the front A/F sensor output.

Fig 3: Identifying A/F Sensor Efficiency
Courtesy of SUBARU OF AMERICA, INC.

ENABLE CONDITION

Secondary Parameters	Enable Conditions
Idle switch off after coolant temp 71°C (160°F)	180 seconds
Closed loop control continuous 5 seconds	Not in standstill
Vehicle speed	24 - 140 km/h (15 - 87 MPH)
Injector pulse (AT)	0.832 -4.096 milliseconds
(MT)	0.384 -3.840 milliseconds
Closed loop control with rear oxygen sensor	Operating
Engine misfire during 400 ignitions	"LESS THAN EQUAL TO" 5 times
Lambda	0.625 - 1.375
Engine speed (AT)	600 - 3000 rpm
(MT)	1000 - 3300 rpm
Neutral position	OFF
Engine coolant temperature	"GREATER THAN EQUAL TO" 5°C (167°F)
Throttle position (AT)	< 14.72°
(MT)	< 17.60°
Average AIA during diagnosis	"GREATER THAN EQUAL TO" 35.2 kg/h
Integrated fluctuating AIA during diagnosis	> 160 kg/h
After fuel shut-off finished	1 second or more
Accumulated amount of intake air (AIA) (start condition)	0.84 kg (1.85 lb)/60 seconds
Vehicle speed (continuous 80 seconds) (start condition)	> 20 km/h (12 MPH)
Injector pulse (continuous 40 seconds) (start condition)	1.34 - 4.22 milliseconds
Vehicle speed (continuous 70 seconds) (start condition)	"GREATER THAN EQUAL TO" 74 km/h (46 MPH)

GENERAL DRIVING CYCLE
Perform the diagnosis once with a constant vehicle speed more than 74 km/h (46 MPH).

DIAGNOSTIC METHOD

Output fluctuation accumulate method

The accumulated value of front oxygen (A/F) sensor output changes and the accumulated value of rear oxygen sensor output changes are determined, and the diagnostic value is calculated using the following formula.

Diagnostic value = Accumulated value of rear oxygen sensor output changes / Accumulated value of front oxygen (A/F) sensor output changes

For example

Fig 4: Output Fluctuation Accumulate Method Graf

Courtesy of SUBARU OF AMERICA, INC.

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

Judgment Value

JUDGMENT VALUE

Malfunction Criteria	Threshold Value
Accumulated variation of output Voltage of rear oxygen sensor per 50 milliseconds divided by accumulated Variation of output voltage of front lambda sensor per 50 milliseconds	"GREATER THAN EQUAL TO" .5

Time Needed for Diagnosis: 45 seconds

Malfunction Indicator Light Illumination: Twice continuous driving cycles.

DTC CLEAR CONDITION
- When the OK idling cycle is completed 40 times in a row
- When "Clear Memory" was performed
MALFUNCTION INDICATOR LIGHT CLEAR CONDITION
- When the OK driving cycle is 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 diagnosis value and trouble standard value. (For test mode $06)