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DTC P0421: Warm Up Catalyst Efficiency Below Threshold
 
 
 
 
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 set 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 (rear) heater monitor
  • Heated oxygen sensor offset voltage monitor
  • Misfire monitor
  • Fuel system monitor
  • Heated oxygen sensor (rear) feedback control system
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
  • Fuel tank pressure sensor
 
 
DTC SET CONDITIONS
Logic Flow Chart
 
 
Check Conditions
  • Engine speed is 3,500 r/min or lower.
  • Mass airflow sensor output is between 9 and 50 g/sec.
  • More than 3 seconds have elapsed after the above mentioned two conditions have been met.
  • Under the closed loop air/fuel ratio control.
  • Barometric pressure is higher than 76 kPa (22.4 in.Hg).
  • Engine coolant temperature is higher than 65°C (149°F).
  • Intake air temperature is higher than -10°C (14°F).
  • Vehicle speed is higher than 1.5 km/h (1.0 mph).
  • The ECM monitors the maximum 15 times per drive cycle under these conditions.
  • 5 seconds or more have passed since the estimated catalytic converter temperature is higher than 652°C (1,206°F).
  • Short-term fuel trim is higher than -25 percent and lower than +25 percent.
  • The linear air-fuel ratio sensor rich/lean switching frequency is more than 8 times.
Judgment 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 the specified value or more.
    noteThe specified value varies depending on the average air flow rate.
noteMonitoring stops when the estimated catalytic converter temperature is 600°C (1,112°F) or lower.
 
 
FAIL-SAFE AND BACKUP FUNCTION
  • None.
 
 
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.
  • ECM failed.

 
 
DIAGNOSIS
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 .
(3) Check the diagnostic trouble code (DTC).
Is DTC P0421 set?
Replace the ECM (Refer to Removal and Installation ).
The inspection is complete.