Code 21, Oxygen Sensor Circuit Open Or Short Circuit (CALIF.): Notes
The Oxygen (O2) sensor determines whether fuel mixture is rich or lean by detecting the concentration of oxygen in the exhaust gas. The O2 sensor is heated and is controlled by the ECM. When intake air volume is low (exhaust gas temperature low), current flows to heat O2 sensor for accurate detection.
When O2 sensor is cold, the heater on time is longer (warming up sensor). When O2 sensor heater gets hot, the heater on time is shortened (reducing sensor temperature). The O2 sensor is provided with a one volt signal from ECM terminal "OX+". The sensor varies this 1-volt signal and sends this varying voltage to the ECM through ECM terminal "OX1".
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- This checks for an intermittent Code 21. Code 21 is set under the following conditions:
- Vehicle must be driven at speeds greater than 40 MPH.
- Turn ignition off and then on. Run engine at greater than 1900 RPM for one minute. If fault is present, Code 21 will set.
- This test determines if O2 sensor is varying voltage from .1-1.0 volt.
- This test determines if O2 sensor heater circuit is working.