P0421

DTC P0421: Warm Up 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.

DESCRIPTIONS OF MONITOR METHODS
Front and rear oxygen sensor rich/lean switching frequencies are nearly equal.

MONITOR EXECUTION
Continuous

MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)
Other Monitor (There is no temporary DTC stored in memory for the item monitored below)
- Heated oxygen sensor (front) monitor
- Heated oxygen sensor (rear) monitor
- Heated oxygen sensor heater (front) monitor
- Heated oxygen sensor heater (rear) monitor
- Misfire monitor
- Fuel system monitor
- Air/fuel ratio feedback monitor
Sensor (The sensor below is determined to be normal)
- Volume airflow sensor
- Engine coolant temperature sensor
- Intake air temperature sensor
- Barometric pressure sensor
- Throttle position sensor

DTC SET CONDITIONS

Logic Flow Chart:

Check Conditions
- Engine speed is lower than 3,000 r/min.
- Volume airflow sensor output frequency is between 63 and 169 Hz.
- The throttle valve is open.
- More than 3 seconds has elapsed after the three above-mentioned conditions have been met.
- Intake air temperature is higher than 10° C (14 |F).
- Barometric pressure is higher than 76 kPa (22.4 in.Hg).
- Under the closed loop air/fuel ratio control.
- Vehicle speed is 1.5 km/h (1 mph) or more.
- The ECM or PCM monitors for this condition for 7 cycles of 10 seconds each during the drive cycle.
- Short-term fuel trim is higher than 25 percent and lower than +25 percent.
- The cumulative volume airflow sensor output frequency is higher than 4,000 Hz.

Judgment Criteria
- The frequency ratio of heated oxygen sensor (rear) and heated oxygen sensor (front) is more than 0.8.

OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Procedure 3 - Catalytic Converter Monitor .

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 leaks.

Q: Are there any abnormalities?

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

STEP 2. Using scan tool MB991502, check data list item 5 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 site 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 P0136 - Heated Oxygen Sensor Circuit (sensor 2), DTC P0137 - Heated Oxygen Sensor Circuit Low Voltage (sensor 2), DTC P0138 - Heated Oxygen Sensor Circuit High Voltage (sensor 2), DTC P0139 - Heate 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 P0130 - Heated Oxygen Sensor Circuit (sensor 1), DTC P0131 - Heated Oxygen Sensor Low Voltage (sensor 1), 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 alternate15 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 Diagnostic Function - OBD-II Drive Cycle - Procedure 3 - Catalytic Converter Monitor.
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

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 Diagnostic Function - OBD-II 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 Diagnostic Function - OBD-II 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.