Code No. P0421: 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 front and rear oxygen sensor signals.

Check Conditions

Engine speed is less than 3,500 r/min <M/T> or 3,000 r/min <CVT>.
Accelerator pedal is open.
Air flow sensor output is more than 6.4 g/sec and is less than 65 g/sec.
More than 3 seconds have elapsed after the above mentioned 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 5 cycles of 10 seconds each during the drive cycle.
Short-term fuel trim is more than -25 % and is less than +25 %.
The cumulative air flow is more than 340 g.

Judgement 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 0.85.

note

The specified value varies depending on the average air flow rate.

DIAGNOSIS PROCEDURE

After replacing the catalytic converter, reconfirm whether the diagnosis code is set.

	Replace the engine-ECU.

	Check end.