Troubleshooting

P0420 - Catalyst System Efficiency Below Threshold

CIRCUIT DESCRIPTION

The ECM compares the waveform of the oxygen sensor located before the catalyst with the waveform of the oxygen sensor located behind the catalyst to determine whether or not catalyst performance has deteriorated. Air fuel ration feedback compensation keeps the waveform of the oxygen sensor before the catalyst repeatedly changing back and forth from rich to lean. If the catalyst is functioning normally, the waveform of the oxygen sensor behind the catalyst switches back and forth from rich to lean much more slowly than the waveform of the oxygen sensor before the catalyst. But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.

DTC Detecting Condition:

Confirmation Driving Pattern





a. Connect the TOYOTA hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals Ox1, OX2 and E1 of the ECM connector.
b. Start engine and warm it up with all accessories switched OFF until water temperature is stable.
c. Drive the vehicle at 55 mph or more for 5 minutes or more.
d. After confirming that the waveforms of the oxygen sensor, bank 1 sensor 1 (OX1), oscillate around 0.5 V during feedback to the ECM, check the waveform of the oxygen sensor, bank 1 sensor 2 (OX2).




Inspection Procedure

Step 1 - 3:

Step 4 - 5: