A3: Check For O2 Sensor Failure

Run the engine for a period to warm the sensor & then maintain a steady speed of approximately 2500 rpm
With EM021 disconnected, measure the voltage between EM021/003 & EM021/004.

Is the voltage between 600mV & 1.0 Volt?

Yes
Go to A4 A4: Check For O2 Sensor Failure

No
Renew O2 sensor; reconnect plug(s)
Go to A6 A6: Stop