Code No. P0431: Warm Up Catalyst Malfunction (left bank) <Except Euro 2>

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

Check Conditions

Engine speed is less than 3,500 r/min.
Accelerator pedal is open.
Air flow is more than 7.7 - 46 g/sec and is less than 60 g/sec.
More than 3 seconds have elapsed after the abovementioned three conditions have been met.
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 3 cycles of 12 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.

Judgement Criterion

When the monitoring for 12 seconds is carried out 5 times, the frequency ratio of rear and front is more than 0.7.

DIAGNOSIS PROCEDURE

	Go to Step 2.

	Repair.

	Go to Step 3.

	Perform the diagnosis code classified check procedure for the sensor that has shown an abnormal data value. Inspection chart for diagnosis code (Refer to ).

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 left bank oxygen sensor (front).

After replacing the left bank oxygen sensor (rear), reconfirm whether the diagnosis code is stored.

	Go to Step 5.

	Check end.

After replacing the catalytic converter (left bank), reconfirm whether the diagnosis code is stored.

	Replace the engine-ECU.

	Check end.