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 rich condition is detected.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
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.