Troubleshooting

DTC 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 after the catalyst to determine whether or not 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 is functioning 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, it indicates that catalyst performance has deteriorated.





DETECTING CONDITION

HINT: U.S.A. and Canadian vehicles does the MIL light up when a malfunction is detected.

WIRING DIAGRAM





CONFIRMATION ENGINE RACING PATTERN
(1) Connect the LEXUS hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals OXR, OXL, 0XS and E1 of ECM.
(2) Start engine and warm it up with all accessories switched OFF until water temp. is stable.
(3) Race the engine at 2,500 ~ 3,000 rpm for about 3 min.
(4) After confirming that the waveform of the heated oxygen sensor, bank 1, 2 sensor 1 (OXR, OXL), oscillate around 0.5 V during feedback to the ECM, check the waveform of the heated oxygen sensor, bank 1 sensor 2 (OXS).





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

Step 1:

Step 2:

Step 3:

INSPECTION PROCEDURE