DTC P0421: Warm Up Catalyst Efficiency Below Threshold (bank 1)

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 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 69 and 169 Hz.
Intake air temperature is higher than -10°C (14°F).
Barometric pressure is higher than 76 kPa (22.4 in.Hg).
The throttle valve is open.
Under the closed loop air / fuel ratio control.
Vehicle speed is 1.5 km/h (0.93 mph) or more.
The PCM monitors for this condition for 7 cycles of 12 seconds each during the drive cycle.
Short-term fuel trim is higher than -30 percent and lower than +25 percent.
The cumulative volume airflow sensor output frequency is higher than 4,000 Hz.

Judgment Criteria
• The right bank heated oxygen sensor (rear) signal frequency divided by right bank heated oxygen sensor (front) signal frequency = 0.75 or more.
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:
• Right bank side catalytic converter deteriorated.
• Right bank heated oxygen sensor failed.
• Exhaust leaks
• PCM failed.