Code No. P0420: Catalyst Malfunction

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

Check Conditions

Engine speed is 3,000 r/min or lower.
The accelerator pedal is depressed.
Mass air flow rate output is 8 g/s or higher.
3 seconds or more elapsed after the above mentioned conditions were satisfied.
Intake air temperature is -10°C or higher after the engine is started.
Barometric pressure is 76 kPa or higher.
Under the closed loop air/fuel ratio control.
Vehicle speed is 1.5 km/h or higher.
For 10 seconds per cycle, the engine-ECU monitors 7 cycles of this condition during the drive cycle.
Short-term fuel trim is between -25% or higher and +25% or lower.
The cumulative air flow sensor output is 2,931 g or higher.

Judgment Criterion

The oxygen sensor (rear) signal frequency divided by oxygen sensor (front) signal frequency is 0.8 or more.

DIAGNOSIS PROCEDURE

	Go to Step 2 .

	Repair for leakage of exhaust emission from exhaust manifold.

	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 Code ).

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).

	Go to Step 4 .

	Replace the oxygen sensor (front).

	Check end.

	Go to Step 5 .

	Replace the engine-ECU then perform the variant coding (Refer to GROUP 00 - Precautions Before Service - How to Perform Variant Coding ).

	Check end.