DTC P0420: Catalyst System Efficiency Below Threshold (Forester)
- Perform test procedure. See TESTING PROCEDURE . If DTC P0420 is only DTC set, go to next step. If any other DTCs exist, diagnose and repair those DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . Do not diagnose DTC P0420. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .
- Check for exhaust leaks or air suction caused by loose or missing nuts and bolts, or damage to exhaust system. Check especially between cylinder head and front exhaust pipe, between front exhaust pipe and front catalytic converter, and between front and rear catalytic converters. If no damage is found, go to next step. If damage is found, repair or replace exhaust system. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .
- Separate rear catalytic converter from rear exhaust pipe. If there is no damage at rear face of rear catalytic converter, go to next step. If there is damage at rear face of rear catalytic converter, replace front and rear catalytic converters. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .
- Remove front catalytic converter. If there is no damage to front or rear faces of front catalytic converter, go to next step. If there is damage at front or rear faces of front catalytic converter, replace front catalytic converter. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .
- Check rear heated oxygen sensor shield wire. If shield wire is okay, go to next step. If shield wire is not okay, repair as required. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .
- Turn ignition off. Disconnect 30-pin ECM electrical harness connector B136. Using a DVOM, measure resistance of Black/Red wires between 30-pin ECM electrical harness connector B136 terminals No. 21 and 22 and good engine ground point. If resistance is less than 5 ohms, contact authorized Subaru dealership because probable cause is deterioration of multiple parts. If resistance is not less than 5 ohms, repair open circuit and check for poor terminal contact in all connectors between ECM and engine ground point. Engine ground point is located at center rear of engine. Clear memory. See CLEARING DIAGNOSTIC TROUBLE CODES . Validate repair by performing appropriate drive cycle. See DRIVE CYCLES .