Circuit Description

A "closed loop" air/fuel metering system is utilized for driveability, fuel economy and emission control. While in closed loop, VCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change in fuel delivery can be monitored using a scan tool and is indicated by Long Term (LT) and Short Term (ST) fuel trim values. Ideal fuel trim is about 128. If a lean condition is present, VCM will increase fuel, resulting in a fuel trim value greater than 128. If a rich condition is present, VCM will reduce fuel, resulting in a fuel trim value less than 128. DTC will set if an excessively rich condition is detected.

DTC will set when the following conditions are present:

• No IAC DTCs are set.
• No HO2S DTCs are set.
• No TP sensor DTCs are set.
• No MAP sensor DTCs are set.
• No EGR DTCs are set.
• No EVAP DTCs are set.
• No ECT sensor DTCs are set.
• No MAF sensor DTCs are set.
• No IAT sensor DTCs are set.
• No VSS DTCs are set.
• No system voltage DTCs are set.
• No misfire DTCs are set.
• Throttle position is less than 69.9 percent.
• Engine speed is greater than 575 RPM but less than 4500 RPM.
• BARO is greater than 70 kPa.
• ECT is greater than 32°F (0°C) but less than 208°F (98°C).
• MAP is greater than 20 kPa but less than 98.9 kPa.
• IAT is greater than -4°F (-20°C) but less than 158°F (70°C).
• Airflow greater than 3 gm/s but less than 150 gm/s.
• Vehicle speed is less than 85 MPH.
• Average short term fuel trim is less than 115.