Circuit Description

PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery is indicated by the long and short term fuel trim values. Ideal fuel trim value is about 128 (zero percent).

If HO2S signals are indicating a lean condition, the PCM will add fuel, resulting in fuel trim values greater than zero percent. If a rich condition is detected, fuel trim values will be less than zero percent, indicating that PCM is reducing amount of fuel delivered.

For duplication of DTC, ensure:

• No active misfire, fuel injector circuit, EVAP system, fuel trim, TP, IAT, MAP, EGR, ECT, MAF or CKP sensor DTCs are set.
• ECT is 68-230°F (20-110°C).
• IAT is 0-149°F (-18 to 65°C).
• TP sensor angle is less than 90 percent.
• Vehicle speed is less than 75 MPH.
• Engine speed is 600-4000 RPM.
• Air flow between 3-150 grams per second.
• BARO is greater than 70 kPa.