Troubleshooting

DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

MONITOR DESCRIPTION

DTC Detecting Condition:

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunction Thresholds:

Component Operating Range:

Monitor Result (Mode 06 Data):

The ECM uses sensors mounted before and after the three-way catalyst (TWC) to monitor its' efficiency. The first sensor, an air fuel ratio sensor (A/F sensor 1), sends pre-catalyst A/F ratio information to the ECM.

The second sensor, a heated oxygen sensor (O2S) sends post-catalyst information to the ECM. The ECM compares these two signals to judge the efficiency of the catalyst and the catalyst's ability to store oxygen.

During normal operation, the TWC stores and releases oxygen as needed. The capacity to store oxygen results in a low variation in the post-TWC exhaust stream as shown.

If the catalyst is functioning normally, the waveform of the heated oxygen sensor located behind the catalyst slowly switches back and forth between rich and lean.

When the waveform of the heated oxygen sensor located behind the catalyst alternates frequently between rich and lean, it indicates that catalyst performance has deteriorated. As the catalyst efficiency degrades, its ability to store oxygen is reduced and the catalyst output becomes more variable.

When the running the monitor, the ECM compares sensor 1 and sensor 2 signals over a specific time to determine catalyst efficiency. The ECM begins by calculating the signal length for both sensors.

The ECM uses the signal length values of rear oxygen sensor output voltage. If the signal length is greater than the failure threshold (which varies with A/F sensor signal length), the ECM interprets this as a fault in the catalyst. The ECM will turn on the MIL and a DTC will be set.

HINT:
- Bank 1 refers to the bank that includes cylinder No.1.
- Bank 2 refers to the bank that does not include cylinder No.1.
- Sensor 1 refers to the sensor closest to the engine assembly.
- Sensor 2 refers to the sensor farthest away from the engine assembly.

CONDITIONING THE A/F SENSOR AND HEATED OXYGEN SENSOR FOR TESTING




a. Connect the OBD II scan tool or the hand-held tester to the DLC3.
b. Start the engine and warm it up with all the accessories switched OFF until the engine coolant temperature is stable.
c. Run the engine at 2,500 to 3,000 rpm for about 3 minutes.
d. When running the engine at 3,000 rpm for 2 seconds and 2,000 rpm for 2 seconds alternately, check the waveform of the oxygen sensor (bank 1 sensor 2).

Step 1:

Step 2 - 4:

INSPECTION PROCEDURE

HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data records the engine conditions when a 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.