DTC 21 (CALIFORNIA Emissions), O2S, Open Or Shorted Circuit: Notes

NOTE: For DTC 21 (Federal Emissions) see DTC 21 (FEDERAL EMISSIONS) chart.

When Oxygen sensor (O2S) reaches operating temperature, it produces a varying voltage. Voltage will vary from 0.1 volt (exhaust is lean) to 1.0 volt (exhaust is rich). ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. DTC 21 will set if sensor input voltage at ECM is 0.35-0.70 volt for 60 seconds or more with engine speed greater than 1500 RPM, and coolant temperature is greater than 176°F (80°C).

NOTE: Test numbers refer to test numbers on diagnostic chart.

Checks for a faulty O2S.
Checks for an open in Black wire between ECM and O2S.
Checks for an open in Brown wire between ECM and G106.
Checks for a short to ground in Black wire between ECM and O2S and for a faulty ECM.