P0421

TECHNICAL DESCRIPTION
- The signal from the rear heated oxygen sensor differs from the front heated oxygen sensor. That is because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the front heated oxygen sensor becomes similar to the rear heated oxygen sensor.
- The ECM or PCM compares the output of the front and rear heated oxygen sensor signals.

DTC SET CONDITIONS

Check Conditions
- Engine speed is lower than 3,000 rpm.
- The throttle valve is open.
- Volume air flow sensor output frequency is between 69 and 169Hz.
- More than 3 seconds have elapsed after the above mentioned three conditions have been met.
- Intake air temperature is higher than -10 °C (14 °F).
- Barometric pressure is higher than 76 kPa (11 psi).
- Under the closed loop air / fuel ratio control.
- Vehicle speed is 1.5 km/h (0.93 mph) or more.
- The ECM or PCM monitors for this condition for 7 cycles of 12 seconds each during the drive cycle.
- Short-term fuel trim is higher than -30 percent and lower than +25 percent.
- The cumulative volume air flow sensor output frequency is higher than 4,000 Hz.

Judgment Criteria
- The right bank heated oxygen sensor (rear) signal frequency divided by right bank heated oxygen sensor (front) signal frequency = 0.8 or more.

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Right bank side catalytic converter deteriorated.
- Right bank heated oxygen sensor failed.
- ECM failed.
- PCM failed.

DIAGNOSIS

Required Special Tool:
- MB991502: Scan Tool (MUT-II)

STEP 1. Check the exhaust leaks.

Q: Are there any abnormalities?

YES: Go to Step 2.
NO: Repair it. Then go to Step 7.

STEP 2. Using scan tool MB991502, check data list item 69: Heated Oxygen Sensor Bank 1, Sensor 2 (right rear).

CAUTION
To prevent damage to scan tool MB991502, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991502.

1. Connect scan tool MB991502 to the data link connector.
2. Start the engine and run at idle.
3. Set scan tool MB991502 to the data reading mode for item 69, Heated Oxygen Sensor Bank 1, Sensor 2 (right rear).
- Warming up the engine. When the engine is revved, the output voltage should repeat 0 volt and 0.6 to 1.0 volt alternately.
4. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 3.
NO: Refer to DTC P0136 - Heated Oxygen Sensor Circuit (bank 1, sensor 2), DTC P0137 - Heated Oxygen Sensor Circuit Low Voltage (bank 1, sensor 2), DTC P0138 - Heated Oxygen Sensor Circuit High Voltage (bank 1, sensor 2), DTC P0139 - Heated Oxygen Sensor Circuit Slow Response (bank 1, sensor 2).

STEP 3. Using scan tool MB991502, check data list item 39: Heated Oxygen Sensor Bank 1, Sensor 1 (right front).

1. Start the engine and run at idle.
2. Set scan tool MB991502 to the data reading mode for item 39, Heated Oxygen Sensor Bank 1, Sensor 1 (right front).
- Warming up the engine. When the engine is revved, the output voltage should be 0.6 to 1.0 volt.
3. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 4.
NO: Refer to DTC P0130 - Heated Oxygen Sensor Circuit (bank 1, sensor 1), DTC P0131 Heated Oxygen Sensor Circuit Low Voltage (bank 1, sensor 1), DTC P0132 - Heated Oxygen Sensor Circuit High Voltage (bank 1, sensor 1), DTC P0133 - Heated Oxygen Sensor Circuit Slow Response (bank 1, sensor 1), DTC P0134 - Heated Oxygen Sensor No Activity Detected (bank 1, sensor 1).

STEP 4. Using scan tool MB991502, check data list item 39: Heated Oxygen Sensor Bank 1, Sensor 1 (right front).

1. Start the engine and run at idle.
2. Set scan tool MB991502 to the data reading mode for item 39, Heated Oxygen Sensor Bank 1, Sensor 1 (right front).
3. keep the engine speed at 2,000 rpm.
- 0 - 0.4 and 0.6 - 1.0 volt should alternate 15 times or more within 10 seconds.
4. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 5.
NO: Replace the right bank heated oxygen sensor (front). Then go to Step 7.

STEP 5. Test the OBD-III drive cycle.

1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis OBD-III Drive Cycle - Procedure 3 - Catalytic Converter Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES: Replace the right bank side catalytic converter. Then go to Step 6.
NO: The inspection is complete.

STEP 6. Test the OBD-III drive cycle.

1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis - OBD-III Drive Cycle - Procedure 3 - Catalytic Converter Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES: Replace the ECM or PCM. Then go to Step 7.
NO: The inspection is complete.

STEP 7. Test the OBD-II drive cycle.

1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis - OBD-III Drive Cycle - Procedure 3 - Catalytic Converter Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES: Retry the troubleshooting.
NO: The inspection is complete.