B4: Check For O2 Sensor Failure

Run the engine for a period to warm the sensor and then maintain a steady speed of approximately 2500 rpm.
Measure the voltage between EM021/003 and EM021/004.

Is the voltage between 600mV and 1.0 Volt?

Yes
GO to B5, B5: Check For O2 Sensor Failure

No
Renew O2 sensor, reconnect plug(s).
GO to B8, B8: Stop