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
PCM monitors the HO2S signal voltage and adjust fuel delivery based on signal voltage when system in "closed loop". A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim is about zero percent. If HO2S signal is indicating a lean condition, PCM will add fuel, resulting in a fuel trim greater than zero percent (100 percent maximum lean). If rich condition is detected, fuel trim values will be less than zero percent (-99 percent maximum rich), indicating that PCM is reducing the amount of fuel delivered. If an excessive lean condition is detected, DTC will set.
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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.