Troubleshooting

DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

CIRCUIT DESCRIPTION

DTC Detecting Condition:

The ECM compares the waveform of the A/F sensor located in front of the catalyst with the waveform of the heated oxygen sensor located behind the catalyst to determine whether or not the catalyst performance has deteriorated.

Air-fuel ratio feedback compensation keeps the waveform of the A/F sensor in front 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 behind the catalyst switches back and forth between rich and lean much more slowly than the waveform of the A/F sensor in front of the catalyst.

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

CONFIRMATION DRIVING PATTERN




a. Connect the hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals HAF1A, HAF2A, OX1B, OX2B and E1 of the ECM connector.
b. Start the engine and warm it up with all the accessories switched OFF until the water temperature is stable.
c. Race the engine at 2,500 - 3,000 rpm for about 3 min.
d. After confirming that the waveforms of the A/F sensor (bank 1, 2 sensor 1 (AF1A, AF2A)) which oscillates 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 sensor 2 (OX1B)) is almost the same as that of the A/F sensor (bank 1, 2 sensor 1 (AF1A, AF2A)) on the left.
- There are some cases that, the MIL may either light up or not light up even though a malfunction exists.

Step 1 - 3:

Step 4:

INSPECTION PROCEDURE

HINT: Read freeze frame data using the 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.