DTC 25

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 25 will set if either of the following condition are met:

(1) 02S voltage input at the ECM is less than 0.45 volts for 90 seconds or more with the 02S warm and engine speed above 1,500 rpm, vehicle speed below 100km/h (60 mph), and with engine coolant temperature (ECT) above 50° C (122° F).

(2) When the engine speed varies by more than 15 rpm over the preceding crank angle period during a period of 30 seconds while idling with the ECT above 50° C (122° 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.

A lean air/fuel condition may also be caused by contaminated fuel (water in the fuel), low fuel pressure (leakage somewhere in the fuel system, restricted fuel filter or injectors or a faulty fuel pressure regulator) or even vacuum and exhaust leaks.

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.