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Lean ConditionRich Condition
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Front O2 sensor reports excess oxygen/lean condition *Front O2 reports low oxygen/rich condition *
* Narrow band or wide band
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PCM responds by increasing fuel trim, commanding injectors to:PCM responds by decreasing fuel trim, commanding injectors to:
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Increase fuel injector pulse widthDecrease fuel injector pulse width
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If front 02 reporting true:
If front 02 reporting false:
If front 02 reporting true:
If front 02 reporting false:
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Once PCM achieves stoicheometry:
PCM will not achieve closed loop stoicheometry
Once PCM achieves stoicheometry:
PCM will not achieve closed loop stoicheometry
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LTFT will show positive
LTFT will max out positive
LTFT will show negative
LTFT will max out negative
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STFT will show near 0
STFT will max out positive
STFT will show near 0
STFT will max out negative
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Engine may run well
Engine may run rough/misfire/stall in closed loop
Engine may run well
Engine may run rough/misfire/stall in closed loop
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Engine may return to open loop/substituted valuesEngine may return to open loop/substituted values
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If MAF under reporting incoming air:If MAF over reporing incoming air:
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PCM responds by decreasing fuel injector pulse width.PCM responds by increasing fuel injector pulse width.
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However, O2 sensor detects excess oxygen.
However, O2 sensor detects low oxygen.
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PCM sees conflicting signals from O2 & MAF, so it looks at Throttle Position Sensor & Accelerator Pedal Position Sensor, (some vehicles also look at Wheel Speed Sensors) to compare with MAF's reported incoming air. Illumiates Malfunction Indicator Light with suspected fault. Engine may return to open loop/substituted values. PCM sees conflicting signals from O2 & MAF, so it looks at Throttle Position Sensor & Accelerator Pedal Position Sensor, (some vehicles also look at Wheel Speed Sensors) to compare with MAF's reported incoming air. Illumiates Malfunction Indicator Light with suspected fault. Engine may return to open loop/substituted values.
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If MAP under reporting manifold air:If MAP over reporting manifold air:
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Same result as MAF above?
Same result as MAF above?
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