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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
  • Air-fuel ratio feedback control system monitor
Sensor (The sensor below is determined to be normal)
  • Mass airflow sensor
  • Engine coolant temperature sensor
  • Intake air temperature sensor 1
  • 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 depressed.
  • Mass airflow is between 8 and 45 g/sec.
  • More than 3 seconds have elapsed after the above-mentioned three conditions have been met.
  • More than 70 seconds have passed since the engine starting sequence was completed.
  • Heated oxygen sensor (rear) output voltage with ground offset is more than 0.4 volt.
  • 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 ECM monitors the maximum 8 times per drive cycle under these conditions.
  • Short-term fuel trim is higher than -25 percent and lower than +25 percent.
  • The cumulative mass airflow is higher than 1,638 g.
  • The linear air-fuel ratio sensor rich/lean switching frequency is more than 5 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.
 
 
FAIL-SAFE AND BACKUP FUNCTION
  • None.
 
 
OBD-II DRIVE CYCLE PATTERN
  • Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 17 .
 
 
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
  • Catalytic converter deteriorated.
  • ECM failed.

 
 
DIAGNOSIS
STEP 1. Replace the exhaust front pipe (integrated with catalytic converter).
(1) Replace the exhaust front pipe (integrated with catalytic converter).
(2) Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 17 .
(3) Check the diagnostic trouble code (DTC).
Is DTC P0421 set?
Replace the ECM (Refer to Removal and Installation ).
The inspection is complete.