DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD
ECM compares waveform of Air/Fuel (A/F) sensor (located in front of catalytic converter) with waveform of heated oxygen sensor No. 2 (located behind catalytic converter) to determine if converter performance has deteriorated. If both waveforms change at similar rate, catalyst performance has deteriorated. DTC is set when A/F sensor and heated oxygen sensor No. 2 waveforms have same amplitude after vehicle is driven for 5 minutes at 20-50 MPH. Possible causes are:
Ã Exhaust system leak.
Ã Defective A/F sensor.
Ã Defective heated oxygen sensor No. 2.
Ã Defective catalytic converter.
Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.
Diagnosis & Repair
1. If other DTCs are displayed, diagnose and repair those DTCs first and retest system. If only DTC P0420 is displayed, go to next step. 2. Check exhaust system for leaks. If problem exists, repair as necessary. If problem does not exist, go to next step. 3. Check A/F sensor circuits. See DTC P0125: INSUFFICIENT COOLANT TEMPERATURE FOR CLOSED LOOP FUEL CONTROL. If problem exists, repair as necessary. If problem
does not exist, go to next step. 4. Check heated oxygen sensor No. 2 circuits. See DTC P0136: HEATED OXYGEN SENSOR NO. 2 CIRCUIT. If problem exists, repair as necessary. If problem does not exist, replace catalytic converter.
Tuesday, July 22nd, 2008 AT 7:18 PM