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
The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7:1 air/fuel ratio under normal driving conditions. PCM can make fuel corrections based on this input, by increasing or decreasing injector pulse width (fuel trim). When certain conditions have been met to begin learning fuel control (ECT, closed loop, etc.), the PCM will use the short term fuel trim cell to update one of the 4 long term cells (idle, decel, cruise, accel) being used.
During every drive cycle (trip) PCM will use and update the idle and cruise long term cells and will keep track of how long the control system is operating in these cells. After about 2 minutes of being in each of these cells, the fuel control system will enable EVAP purge (purge on) and the purge long term cells will be used. If average of the idle and cruise fuel cells is 147 and the short fuel trim cell is currently over 128, a lean fuel system DTC will set. This test is performed before the purge cells are used.
DTC will set if the averaged long term idle and cruise fuel trim cells are 147 and the current short term fuel trim cell is greater than 128 for 3 seconds when PCM is using the non-purge long term fuel trim cells, system is in closed loop, coolant temperature is 140-239°F (60-115°C), airflow is 2-80 gm/s, MAP at 3-12 psi (.2-.8 kg/cm2 ), RPM at 500-4000 RPM, TP is less than 75 percent open, BARO at greater than 10 psi (.7 kg/cm2 ), IAT -22°F (-30°C) to 176°F (80°C), vehicle speed less than 75 MPH and no ECT, MAP, IAT, TP sensor, VSS, EVAP canister purge, front O2S DTCs or DTC P0125 not present. Fuel control system is continuously checked when these conditions have been met.
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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.
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