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DTC P0420: 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 heater (rear) monitor
  • Heated oxygen sensor offset voltage monitor
  • Misfire monitor
  • Fuel system monitor
  • Heated oxygen sensor (rear) feedback control system 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 less than 3,000 r/min.
  • Accelerator pedal is depressed.
  • Mass airflow rate is between 10 and 45 g/sec.
  • More than 3 seconds have passed 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 more than -10°C (14°F).
  • Barometric pressure is more than 76 kPa (22.4 in.Hg).
  • Under the closed loop air/fuel ratio control.
  • Vehicle speed is more than 1.5 km/h (1.0 mph).
  • The ECM monitors the maximum 3 times per drive cycle under these conditions.
  • Short-term fuel trim is between -25 percent and +25 percent.
  • The cumulative mass airflow is more than 1,638 g.
  • 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 more than the specified value.
    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 4 .
 
 
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
  • Catalytic converter deteriorated.
  • Heated oxygen sensor failed.
  • Exhaust leak.
  • ECM failed.

 
 
DIAGNOSIS
Required Special Tool:
  • MB991958: Scan Tool (M.U.T.-III Sub Assembly)
    • MB991824: V.C.I.
    • MB991827: USB Cable
    • MB991910: Main Harness A
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 P0420 set?
Replace the ECM (Refer to, Removal and Installation ).
The inspection is complete.