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 <M/T> or PCM <A/T> compares the output of the front and rear heated oxygen sensor signals.
DESCRIPTIONS OF MONITOR METHODS
• If the inversion frequency is found to be equal through a comparison of the lean/rich inversion frequency of the heated oxygen sensor (front), the catalyst is determined to have deteriorated.
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
• Misfire monitor
• Fuel system monitor
• Air fuel ratio feedback system monitor

Sensor (The sensor below is determined to be normal)
• Volume airflow sensor
• Barometric pressure sensor
• Intake air temperature sensor
• Engine coolant temperature sensor
• Throttle position sensor
DTC SET CONDITIONS Logic Flow Chart


Check Conditions
Engine speed is lower than 3,400 r/min.
Volume airflow sensor output frequency is at between 94 and 331 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 <M/T> or PCM <A/T> monitors for this condition for 7 cycles of 10 seconds each during the drive cycle.
Short-term fuel trim is at between -25 and +25 percent.
The cumulative volume airflow sensor output frequency is higher than 4,000 Hz.

Judgment Criterion
• The frequency ratio of heated oxygen sensor (rear) and heated oxygen sensor (front) is more than 0.6.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 4 .
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:
• Catalytic converter deteriorated.
• Heated oxygen sensor failed.
• ECM failed. <M/T>
• PCM failed. <A/T>