California Models

DIAGNOSTIC PROCEDURE FOR DTC 21

NOTE: If DTC 21 is output inspect / replace front oxygen Sensor.
If DTC 28 is output, inspect / replace rear oxygen sensor.




1. CHECK VOLTAGE OF TERMINALS HT1, HT2.
^ If OK proceed to step 3.
^ If not OK proceed to step 2.
2. CHECK RESISTANCE OF MAIN OXYGEN SENSOR.
^ If OK check and repair main oxygen sensor heater circuit.
^ If not OK replace main heated oxygen sensor.
3. CHECK OPERATION OF MAIN HEATED OXYGEN SENSOR HEATER.
^ If OK replace main heated oxygen sensor.
^ If not OK check and replace ECM.