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I have attached a description and diagnosis procedure and you most probably need to replace the front catalytic converter as well.
DTC P0421: WARM-UP CATALYTIC CONVERTER EFFICIENCY BELOW THRESHOLD
NOTE: ECM/PCM compares front and rear HO2S output signals to determine whether rear HO2S output signal is beginning to match the front HO2S output signal. As catalytic converter efficiency declines, the switching frequency of front HO2S and rear HO2S signals become more and more alike.
ECM/PCM monitors catalytic converter efficiency for 140 seconds when the following conditions are met:
Engine speed is 3000 RPM or more.
Closed throttle position switch is off.
ECM/PCM is operating in closed loop.
ECM/PCM determines fault exists when rear HO2S switching frequency becomes like front HO2S switching frequency.
Catalytic converter deteriorated.
1. Check if DTC P0133, P0135, P0136, and/or P0141 is set in addition to DTC P0421. If DTC P0133, P0135, P0136 and/or P0141 is set, repair that DTC first. If DTC P0133, P0135, P0136 and/or P0141 is not set, go to next step.
2. Start engine and allow to idle. Inspect for exhaust leaks upstream and downstream of catalytic converter. Inspect for exhaust leaks around front HO2S and rear HO2S. If no leak is found, go to next step. If leak is found, repair exhaust leak as necessary. When repairs are completed, clear DTC and verify operation.
3. Check rear HO2S operation. If problem does not exist, go to next step. If problem exists, perform repairs or replace rear HO2S as necessary. When repairs are completed, clear DTC and verify operation.
4. Turn ignition off. Replace catalytic converter and clear DTC P0421. Road test vehicle and check for DTCs. If DTC P0421 does not reset, repairs and inspection are complete. If DTC P0421 resets, ensure ECM/PCM connector and terminals are clean and tight. If ECM/PCM connector is okay, replace ECM/PCM. When repairs are completed, clear DTC and verify operation.
Thursday, December 18th, 2008 AT 12:00 PM