Code 21, O2 Sensor Circuit/Open Or Short Circuit

ECM supplies approximately .45 volt between ECM terminals OX+ and OX1. 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 does 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.

1. Code 21 will 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.
2. Determines if problem is faulty O2 sensor, or faulty ECM and wiring.
3. Use a 10-megohm DVOM for this test. This checks continuity of Red and Black wires. If Black wire is open, ECM voltage on Black wire will be greater than .6 volt.