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Home >> Subaru >> 2021 >> WRX STI >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System (Diagnostics) (2.5L) (2 Of 4) >> Diagnostic Procedure with Diagnostic Trouble Code (DTC) >> DTC P0420: Catalyst System Efficiency Below Threshold Bank 1 >> Diagnostic Method
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Diagnostic Method

When TWC performance falls, OSC performance also falls. In the diagnosis, malfunction will be detected with estimation of OSC performance by observing the time and air amount, etc. until the rear oxygen sensor voltage starts to rise after more than predetermined time of deceleration fuel cut has passed.

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

JUDGMENT VALUE SPECIFICATION

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

Time needed for diagnosis:  Less than 1 second

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