Code 21, O2 Sensor CKT/Open Or Short CKT
WARNING: This page is about a different car, the 1991 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-.9 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.
- Code 21 will be set if the following conditions are present for 60 seconds: engine is at normal operating temperature, at least 2 minutes have passed since engine start, and O2 signal voltage is steady between 0-1 volt.
- This determines if problem is faulty O2 sensor, or faulty ECM and wiring.
- Use a 10-megohm 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.