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Can a bad REAR O2 sensor cause decreased gas mileage?

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Old 12-09-2009, 11:29 AM   #1
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Member Since: Jan 2007
Location: College Station or Katy Texas
Default Can a bad REAR O2 sensor cause decreased gas mileage?

I have noticed that the gas mileage on my '02 Z is fairly low lately compared to what I should be getting. Cruising at 50mph in 6th gear I am only averaging around 26mpg and it just gets worse from there. Probably about 21.5mpg at 65mph.

For a while now my car has been throwing a P0140 code. Before I figured "It's a rear O2 and those are just for emissions. Who cares?". But now I'm wondering if this may be the cause of my problem somehow. For a while I have just been clearing the code to get my CEL to go off. It is the ONLY code that ever shows up.

Note: I have not noticed any decrease in performance. The car feels just as strong under heavy throttle as ever. Approximately 74,000 miles on the clock. New plugs and wires around this time last year.

Could a bad rear O2 sensor possibly have anything to do with my poor mileage or should I look elsewhere?
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Old 12-09-2009, 12:11 PM   #2
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Location: Charlotte, NC

Not sure if it being in the rear makes a difference, but I do know that in general a bad O2 sensor will hurt your gas mileage.
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Old 12-09-2009, 12:14 PM   #3
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Check my thread on "A funny thing happened on the way to the dyno". It could be the fuel, although your drop in mileage is rather large on a percentage basis. Do you know if Texas switches to the winter blend which contains higher butane levels? If so, mileage will go down. My truck is down for the last few fills which I now attribute to the fuel.
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Old 12-09-2009, 02:12 PM   #4
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NO! The rear 02 sensors ONLY provide the PCM CAT Efficiency data. In NO way are they used to compute air/fuel ratio for the PCM.

They just tell the PCM if you comply with emissions by having properly operating CATs

VETNUTS has some intriguing info and it and his post is an interesting read.

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Old 12-09-2009, 02:12 PM
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