Code 21, Oxygen Sensor Circuit Open Or Shorted Circuit: Notes

The ECM supplies approximately .45 volt between ECM terminals "OX" and "E1". If reading is taken with a 10-megohms DVOM, reading may be as low as .32 volt. The O2 sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .10-.90 volt. The O2 sensor does not produce a voltage until sensor reaches 600°F (316°C). When sensor is less than this temperature, an "Open Loop" condition exists.

1. Code 21 will set when engine is at normal operating temperature, at least 2 minutes have passed since engine start, O2 signal voltage is steady between 0-1 volt and all these conditions are present for 60 seconds.
2. This determines if O2 sensor is problem or if ECM and wiring are faulty.
3. When doing this check, use a 10-megohms DVOM. This checks continuity of Black and Brown wires. If Brown wire is open, ECM voltage on Black wire will be greater than .6 volt.