P0421

DTC P0421: Warm Up Catalyst Efficiency Below Threshold [Vehicles for California]

TECHNICAL DESCRIPTION
- The signal from the heated oxygen sensor (rear) differs from the linear air-fuel ratio sensor, because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the linear air-fuel ratio sensor becomes similar to the heated oxygen sensor (rear).
- The ECM compares the output of the linear air-fuel ratio sensor and heated oxygen sensor (rear) signals.

DESCRIPTIONS OF MONITOR METHODS
Linear air-fuel ratio sensor and heated oxygen sensor (rear) 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)
- Linear air-fuel ratio sensor monitor
- Heated oxygen sensor (rear) monitor
- Linear air-fuel ratio sensor heater monitor
- Heated oxygen sensor heater (rear) monitor
- Heated oxygen sensor offset voltage monitor
- Misfire monitor
- Fuel system monitor
- Heated oxygen sensor (rear) feedback control system monitor

Sensor (The sensor below is determined to be normal)
- Mass airflow sensor
- Engine coolant temperature sensor
- Intake air temperature sensor
- Barometric pressure sensor
- Throttle position sensor
- Accelerator pedal position sensor

DTC SET CONDITIONS







Check Conditions
- Engine speed is less than 3,500 r/min [M/T] or 3,000 r/min [CVT].
- Accelerator pedal is depressed.
- Mass airflow rate is between 10 and 45 g/sec [M/T] or 9 and 45 g/sec [CVT].
- More than 3 seconds have passed after the above-mentioned three conditions have been met.
- Intake air temperature is more than -10 °C (14 °F).
- Barometric pressure is more than 76 kPa (22.4 in.Hg).
- Under the closed loop air/fuel ratio control.
- Vehicle speed is more than 1.5 km/h (1.0 mph).
- The ECM monitors the maximum 5 times [M/T] or 3 times [CVT] per drive cycle under these conditions.
- Short-term fuel trim is between -25 percent and +25 percent.
- The cumulative mass airflow is more than 1,638 g.
- The linear air-fuel ratio sensor rich/lean switching frequency is more than 8 times.

Judgement Criterion
- When the monitoring for 10 seconds is carried out 5 times, the frequency ratio of heated oxygen sensor (rear) and linear air-fuel ratio sensor signals is more than the specified value.

NOTE: The specified value varies depending on the average air flow rate.

FAIL-SAFE AND BACKUP FUNCTION
- None

OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD- II Drive Cycle

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

DIAGNOSIS

Required Special Tool:
- MB991958: Scan Tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.
- MB991827: USB Cable
- MB991910: Main Harness A

STEP 1. Replace the catalytic converter.

1. Replace the catalytic converter.
2. Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD- II Drive Cycle
3. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES : Replace the ECM.

NO : The inspection is complete.