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).
TROUBLE JUDGEMENT
Check Conditions
Engine speed is less than 3,187 r/min.
The accelerator pedal is open.
More than 3 seconds have elapsed after the above mentioned two 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 is less than +25%.
Judgement Criterion
When the monitoring for 10 seconds is carried out 7 times, the frequency ratio of oxygen sensor (rear) and oxygen sensor (front) is more than 0.8.
FAIL-SAFE AND BACKUP FUNCTION
None
PROBABLE CAUSES
Catalytic converter deteriorated
Failed engine-ECU
DIAGNOSIS PROCEDURE
STEP 1. Replace the catalytic converter.
After replacing the catalytic converter, reconfirm whether the diagnosis code is stored.
Replace the engine-ECU.
Check end.