Troubleshooting

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

CIRCUIT DESCRIPTION

DTC Detection Condition:

The ECM compares the waveform of the oxygen sensor located before the catalyst with the waveform of the oxygen sensor located after the catalyst to determine whether or not the catalyst performance has deteriorated.

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

If the catalyst functions normally, the waveform of the oxygen sensor after the catalyst switches back and forth between rich and lean much more slowly than the waveform of the oxygen sensor before the catalyst.

But when both waveforms change at a similar rate, the catalyst performance has deteriorated.

CONFIRMATION ENGINE RACING PATTERN

Confirmation Engine Racing Pattern:

1. Connect the TOYOTA 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 the accessories switched OFF until the 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 (OXL1, OXR1)) which oscillates around 0.5 V during feedback to the ECM, check the waveform of the heated oxygen sensor (bank 1, 2 sensor 2 (OXL2, OXR2)).






HINT: If there is a malfunction in the system, the waveform of the heated oxygen sensor (bank 1,2 sensor 2 (OXL2, OXR2)) is almost the same as that of the heated oxygen sensor (bank 1, 2 sensor 1 (OXL1, OXR1)) as given in the figure.

There are some cases when the MIL may either light up or not light up even though a malfunction exists.

Step 1:

Steps 2 - 4:

INSPECTION PROCEDURE

HINT: Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool, as freeze frame data records the engine conditions when malfunction is detected. When troubleshooting, it 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.