P0421

P0421: Warmup Catalyst Efficiency Below Threshold

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 r/min.
- Volume air flow sensor output frequency is at between 70 and 150 Hz.
- Intake air temperature is higher than -10 °C (14 °F).
- Barometric pressure is higher than 76 kPa (11 psi).
- The throttle valve is open.
- Under the closed loop air/fuel ratio control.
- Vehicle speed is 1.5 km/h (0.93 mph) or more.
- Monitoring time: 12 seconds x 7 times.
- Short-term fuel trim is at between -25 and +25 percent.
- The cumulative volume air flow sensor output frequency is higher than 4,000 Hz.

Judgment Criteria
- The heated oxygen sensor (rear) signal frequency divided by heated oxygen sensor (front) signal frequency = 0.75 or 0.8 or more.

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Catalytic converter deteriorated.
- Heated oxygen sensor failed.
- ECM failed.
- PCM failed.

DIAGNOSIS

Required Special Tools
- 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 59: Heated Oxygen Sensor (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 59, Heated Oxygen Sensor (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 P0137- Heated Oxygen Sensor Circuit Low Voltage (sensor 2), DTC P0138 - Heated Oxygen Sensor Circuit High Voltage (sensor 2), DTC P0139 - Heated Oxygen Sensor Circuit Slow Response (sensor 2).

STEP 3. Using scan tool MB991502, check data list item 11: Heated Oxygen Sensor (front).
1. Start the engine and run at idle.
2. Set scan tool MB991502 to the data reading mode for item 11, Heated Oxygen Sensor (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 P0132- Heated Oxygen Sensor Circuit High Voltage (sensor 1), DTC P0133 - Heated Oxygen Sensor Circuit Slow Response (sensor 1), DTC P0134 - Heated Oxygen Sensor Circuit No Activity Detected (sensor 1).

STEP 4. Using scan tool MB991502, check data list item 11: Heated Oxygen Sensor (front).
1. Start the engine and run at idle.
2. Set scan tool MB991502 to the data reading mode for item 11, Heated Oxygen Sensor (front).
3. Keep the engine speed at 2,000 r/min.
- 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 heated oxygen sensor (front). Then go to Step 7.

STEP 5. Test the OBD-II drive cycle.
1. Carry out a test drive with the drive cycle pattern. Refer to, Procedure 3 Catalytic Converter Monitor and Procedure 6 - Other Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is the DTC P0421 output?

YES: Replace the catalytic converter. Then go to Step 6.

NO: The inspection is complete.

STEP 6. Test the OBD-II drive cycle.
1. Carry out a test drive with the drive cycle pattern. Refer to, Procedure 3 Catalytic Converter Monitor and Procedure 6 - Other Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is the DTC P0421 output?

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, Procedure 3 Catalytic Converter Monitor and Procedure 6 - Other Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is the DTC P0421 output?

YES: Retry the troubleshooting.

NO: The inspection is complete.