Troubleshooting

DTC P0420 Catalyst System Efficiency Below Threshold (Bank 1)

CIRCUIT DESCRIPTION

DTC Detecting Condition:

The ECM compares the waveform of the heated oxygen sensor located forward of the catalyst with that of the heated oxygen sensor located backward of the catalyst to determine whether or not catalyst performance has deteriorated.

Air-fuel ratio feedback compensation keeps the waveform of the heated oxygen sensor forward of the catalyst repeatedly changing back and forth from rich to lean.

If the catalyst is functioning normally, the waveform of the heated oxygen sensor backward of the catalyst switches back and forth between rich and lean much more slowly than the waveform of the heated oxygen sensor before the catalyst.

But when both waveforms change at a similar rate. It indicates that catalyst performance has deteriorated.

CONFIRMATION ENGINE RACING PATTERN




1. Connect the hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals OXL1, OXL2, OXR1, OXR2 and E1 of the ECM connector.
2. Start the engine and warm it up with all accessories switched OFF until engine coolant temperature is stable.
3. Race the engine at 2,500 - 3,000 rpm for about 3 minutes.
4. After confirming that the waveform of the heated oxygen sensor (bank 1, 2 sensor 1 (OX1A, OX2A)), oscillate around 0.5 V during feedback to the ECM, check the waveform of the heated oxygen sensor (bank 1, 2 sensor 2 (OX1B, OX2B)).






HINT:
If there is a malfunction in the system, the waveform of the heated oxygen sensor (bank 1, 2 sensor 2 (OX1B, OX2B)) is almost the same as that of the heated oxygen sensor (bank 1 2 sensor 1 (OX1A, OX2A)).
There are some cases where, even though a malfunction exists, the MIL may either light up or not light up.

Step 1:

Step 2 - 4:

INSPECTION PROCEDURE

HINT: Read freeze frame data using hand-held tester or the OBD II scan tool, as freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, this is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.