Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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 misfire DTCs are set.
- Throttle position is less than 69.9 percent.
- Engine speed between 575-4000 RPM.
- BARO is greater than 70 kPa.
- ECT 167-237°F (75-114°C).
- MAP 22-85 kPa.
- IAT 46-169°F (8-76°C).
- MAF 3-85 gm/s.
- Vehicle speed is less than 85 MPH.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.