P0421

DTC P0421: Warm Up Catalyst Efficiency Below Threshold (cylinder 1, 4)

TECHNICAL DESCRIPTION
- The signal from the rear heated oxygen sensor differs from the front heated oxygen sensor, 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 PCM compares the output of the front and rear heated oxygen sensor signals.

DESCRIPTIONS OF MONITOR METHODS
Front and rear heated 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 (front) heater monitor
- Heated oxygen sensor (rear) heater monitor
- Heated oxygen sensor (front) inactive monitor
- Heated oxygen sensor offset voltage monitor
- Misfire monitor
- Fuel system monitor
- Air/fuel ratio feedback 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

Logic Flow Chart:

Check Conditions
- Engine speed is lower than 3,000 r/min.
- Accelerator pedal is open.
- Mass airflow is between 14 and 45 g/sec.
- 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 (22.4 in.Hg).
- Under the closed loop air/fuel ratio control.
- Vehicle speed is 1.5 km/h (1.0 mph) or more.
- The PCM monitors for this condition for 5 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 mass airflow is higher than 1,638 g.

Judgement Criterion
- The frequency ratio of cylinder 1, 4 heated oxygen sensor (rear) and cylinder 1, 4 heated oxygen sensor (front) is more than 0.6 [California] or 0.7 [Except for California].

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:)
- Cylinder 1, 4 side catalytic converter deteriorated.
- Cylinder 1, 4 heated oxygen sensor failed.
- Exhaust leak.
- PCM failed.

DIAGNOSIS

STEP 1. Replace the cylinder 1, 4 side catalytic converter.

1. Replace the cylinder 1, 4 side 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 PCM. When the PCM is replaced, register the encrypted code.

NO : The inspection is complete.