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Old 12-01-2013, 01:01 AM   #1
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Upstream O2 Sensor Question

Now most Jeeps have 4 O2 sensors, 2 Upstream and 2 Downstream. Ive read somewhere that only the front Upstream O2 Sensor does the adjusting for the fuel mixture and the rear upstream is used in conjunction with the downstream to measure the cats. Ive tried to look for more info about it elsewhere and cannot find any info about it.

Is it true that only the front O2 sensors control the fuel mixture or do they all play a part in it?
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Old 12-01-2013, 05:59 AM   #2
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I don't know about the Jeeps with the 4 O2 sensors . But I asked the same question about my 97 (only 2 sensors) and was told the upstream helps control the fuel mix and the downstream only turned on the C.E.L. . If you replace one do not use a dielectric grease on the contacts it will mess up the readings from the sensors. Maybe someone like Jerry Bransford will chime in on this. HTH

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Old 12-01-2013, 11:05 PM   #3
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Anyone else know?
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Old 12-02-2013, 05:50 AM   #4
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Quote:
Originally Posted by tkfx View Post
Anyone else know?
The upstream sensor, or "feedback", is used for fuel trim, the downstream a catalyst monitor. When graphed, under ideal conditions, the fore sensor will register rapid switching, while the aft sensor a series of shallow, regular peaks and troughs that indicate a lit, functional, healthy catalyst (and therefore a healthy engine management system). They are both interdependent in operation of the malfunction indicator, and compliance of IM240.
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Old 12-02-2013, 05:53 PM   #5
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Quote:
Originally Posted by 1979WagoneerLTD View Post
The upstream sensor, or "feedback", is used for fuel trim, the downstream a catalyst monitor. When graphed, under ideal conditions, the fore sensor will register rapid switching, while the aft sensor a series of shallow, regular peaks and troughs that indicate a lit, functional, healthy catalyst (and therefore a healthy engine management system). They are both interdependent in operation of the malfunction indicator, and compliance of IM240.
Thank you for the great explanation!
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Old 12-04-2013, 01:58 PM   #6
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Quote:
Originally Posted by 1979WagoneerLTD View Post
The upstream sensor, or "feedback", is used for fuel trim, the downstream a catalyst monitor. When graphed, under ideal conditions, the fore sensor will register rapid switching, while the aft sensor a series of shallow, regular peaks and troughs that indicate a lit, functional, healthy catalyst (and therefore a healthy engine management system). They are both interdependent in operation of the malfunction indicator, and compliance of IM240.
Well said brother

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