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Home >> Subaru >> 2018 >> WRX STI >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System (Diagnostics) (2 Of 4) (STI) >> Diagnostic Procedure with Diagnostic Trouble Code (DTC) >> DTC P0420: Catalyst System Efficiency Below Threshold Bank 1 >> Diagnostic Method
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Diagnostic Method

After the enable conditions have been established, calculate the front oxygen (A/F) sensor lambda deviation sum value and the rear oxygen sensor output voltage deviation sum value in every 32 ms x4 times. If the front oxygen (A/F) sensor lambda deviation sum value is the predetermined value or more, calculate the diagnostic value.

If the duration of time while the following conditions are met is within the time indicated, judge as NG.

JUDGMENT VALUE SPECIFICATION (EXCEPT FOR TYPE RA)

Malfunction Criteria Threshold Value
Rear oxygen sensor output voltage deviation sum value/front oxygen (A/F) sensor lambda deviation sum value > 8
JUDGMENT VALUE SPECIFICATION (TYPE RA)

Malfunction Criteria Threshold Value
Oxygen Storage Capacity (OSC) of Catalytic < 0.31g (0.01oz)

Time needed for diagnosis: 

Except for TYPE RA: 30 - 55 s

TYPE RA: Less than 1 s

Malfunction indicator light illumination:  Illuminates when malfunction occurs in 2 continuous driving cycles.