DTC 21

Oxygen Sensor Circuit Open/Shorted:

Oxygen Sensor Circuit:

Circuit Description:
When the oxygen sensor (02S) reaches operating temperature, it produces a varying voltage between 0.10 volts (exhaust is lean) and 1.0 volts (exhaust is rich). The engine control module (ECM) monitors this voltage and determines the concentration of gases in the exhaust. The ECM then uses this input to make fuel corrections to maintain the optimum air/fuel ratio.

DTC 21 will set if the following condition is met:

^ 02S voltage input at the ECM is constant between 0.35 and 0.70 volts for 60 seconds or more with engine speed above 1,500 rpm and engine coolant temperature (ECT) above 80° C (176° F)


Test Description:
1. This checks to see if the 02S is faulty.

2. This checks for an open in the BLK wire between the ECM and 02S.

3. This checks for an open in the BRN wire between the ECM and G106.

4. This checks for a short to ground in the BLK wire between the ECM and 02S, an open in the BRN wire to the 02S on Federal emissions equipped vehicles and for a faulty ECM.

Diagnostic Aids:
Make sure that G106 is clean and tight.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections before component replacement.