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

WARNING: This page is about a different car, the 1992 Geo Prizm. However, it is still accessible from the selected car via links, so may be relevant.

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.