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 lean condition is detected.

Conditions for setting DTC:

• No ECT, EGR, EVAP, HO2S, IAC, IAT, MAF, MAP, TP or VS sensor DTCs are set.
• No system voltage DTCs are set.
• No misfire DTCs are set.
• Throttle position is less than 69.9 percent.
• Engine speed 575-4000 RPM.
• BARO is greater than 70 kPa.
• ECT between 167-237°F (75-114°C).
• MAP between 22-85 kPa.
• IAT between 46-169°F (-8 to 76°C).
• MAF between 3-85 gm/s.
• Vehicle speed is less than 85 MPH.