P0421

P0421: Warmup Catalyst Efficiency Below Threshold (California)

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 examines 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 between 63 and 169 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.
- In closed loop.
- Vehicle speed is 1.5 km/h (0.93 mph) or more.
- Monitoring time: 70 seconds.

Judgment Criteria
- The heated oxygen sensor (rear) signal and heated oxygen sensor (front) signal are similar.

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 Tool:
MB991502: Scan Tool (MUT-II)

STEP 1. Check the exhaust leakage.
1. Are there any cracks, etc., in the exhaust manifold or front exhaust pipe?

If no, go to Step 2.
If yes, repair or replace. Then go to Step 7.

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

STEP 2. Using scan tool MB991502, check data list item 59: Heated Oxygen Sensor (rear).
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).
4. Run in 2nd gear at a wide open throttle.
- The output voltage should be between 0.6 and 1.0 volt after the engine is warmed.

If within specifications, go to Step 3.
If not within specifications, refer to DTC P0136 O2 Sensor Circuit Malfunction (Sensor 2) .

STEP 3. Using scan tool MB991502, check data list item 11: Heated Oxygen Sensor (front).
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 11, Heated Oxygen Sensor (front).
- The output voltage should be between 0.6 and 1.0 volt after racing the engine.

If within specifications, go to Step 4.
If not within specifications, refer to DTC P0130 - O2 Sensor Circuit Malfunction (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.

If within specifications, go to Step 5.
If not within specifications, 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.
2. Read the diagnostic trouble code (DTC).

If DTC P0421 is output, replace the catalytic converter. Then go to Step 6.
If DTC P0421 is not output, 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.
2. Read the diagnostic trouble code (DTC).

If DTC P0421 is output, replace the ECM or PCM. Then go to Step 7.
If DTC P0421 is not output, 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.
2. Read the diagnostic trouble code, and confirm that diagnostic trouble code P0421 does not reset.