Code No. P0420: Catalyst malfunction




FUNCTION



  • The signal from the heated oxygen sensor (rear) differs from the heated oxygen sensor (front). That is because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the heated oxygen sensor (front) becomes similar to the heated oxygen sensor (rear).
  • The engine-ECU compares the output of the front and rear heated oxygen sensor signals.

TROUBLE JUDGMENT


Check Conditions
  • Engine speed is less than 3,500 r/min <M/T>.
  • Engine speed is less than 3,000 r/min <CVT>.
  • Accelerator pedal is open.
  • Air flow sensor output is more than 6.4 g/sec and less than 65 g/sec.
  • More than 3 seconds have elapsed after the above mentioned three conditions have been met.
  • Intake air temperature is more than -10°C.
  • Barometric pressure is more than 76 kPa.
  • Under the closed loop air/fuel ratio control.
  • Vehicle speed is more than 1.5 km/h.
  • The engine-ECU monitors for this condition for 5 cycles of 10 seconds each during the drive cycle.
  • Short-term fuel trim is more than -25 % and less than +25 %.
  • The cumulative air flow is more than 1,638 g.
Judgment Criterion
  • When the monitoring for 10 seconds is carried out 5 times, the frequency ratio of oxygen sensor (rear) and oxygen sensor (front) is more than 0.7.

PROBABLE CAUSE



  • Catalytic converter deteriorated
  • Failed oxygen sensor (front)
  • Failed oxygen sensor (rear)
  • Failed engine-ECU

DIAGNOSIS PROCEDURE


STEP 1. Check for leakage of exhaust emission from exhaust manifold.


Q. Is the check result normal?

Go to Step 2 .
Repair.

STEP 2. M.U.T.-III data list



  • Refer to Data list reference table .

  1. Item AC: Oxygen sensor (front)
  2. Item AD: Oxygen sensor (rear)

Q. Is the check result normal?

Go to Step 3 .
Perform the diagnosis code classified check procedure for the sensor that has shown an abnormal data valve (Refer to Inspection Chart for Diagnosis Codes ).

STEP 3. M.U.T.-III data list



  1. Item AC: Oxygen sensor (front)
OK: 0 - 0.4 and 0.6 - 1.0 volt should alternate 15 times or more within 10 seconds (engine speed at 2,000 r/min).

Q. Is the check result normal?

Go to Step 4 .
Replace the oxygen sensor (front).

STEP 4. Replace the oxygen sensor (rear).



  • After replacing the oxygen sensor (rear), re-check the trouble symptoms.

Q. Is the check result normal?

Check end.
Go to Step 5 .

STEP 5. Replace the catalytic converter.



  • After replacing the catalytic converter, re-check the trouble symptoms.

Q. Is the check result normal?

Replace the engine-ECU.
Check end.