Code 21 (PRIZM) - Oxygen Sensor, Open Or Shorted Circuit: Notes

Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 21 will set if sensor input voltage at ECM is .35-.70 volt for 60 seconds or more with engine speed greater than 1500 RPM, and coolant temperature is greater than 176°F (80°C).

1. Checks for faulty sensor.
2. Checks for open in Black wire between ECM and sensor.
3. Checks for open in Brown wire between ECM and G106.
4. Checks for short to ground in Black wire between ECM and sensor, open in Brown wire to sensor (federal emissions only), and a faulty ECM.