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

ECM supplies approximately .45 volt between ECM terminals OX and E1. If voltage is measured with a 10-megohm 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 .1-1.0 volt.

The O2 sensor will not produce a voltage until sensor reaches 600°F (316°C). When sensor temperature is less than 600°F (316°C), an open loop condition exists.

NOTE: Test numbers refer to test numbers on diagnostic chart.

This step checks for a poor connection or for a faulty ECM. Code 21 will set under the following conditions:
- Engine at normal operating temperature.
- At least 2 minutes have elapsed since engine start-up.
- O2 signal voltage is steady between zero or 1 volt.
- All above conditions are met for about 60 seconds.
Determines if O2 sensor is problem or if ECM or wiring are at fault.