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Old 01-18-2011, 11:02 AM   #1
eliaslincoln
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o2 sensor questions

I dont know a lot about the o2 sensor so can some of you fill me in?

1) How often do they need replaced? (im not running a cat)

2) I have a cheater on the one o2, but would it still need replacing? (how exactly does the cheater work?)

3) Can an old/going bad o2's effect mpg? (this is why im asking questions)

my car has the stock o2's with 93,000 miles.

thanks!


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Old 01-18-2011, 08:45 PM   #2
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no one?
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Old 01-18-2011, 09:17 PM   #3
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My stock o2 needed to be replaced around 130k. The one after the stock cat.
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Old 01-19-2011, 12:50 AM   #4
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Quote:
Originally Posted by eliaslincoln View Post
I dont know a lot about the o2 sensor so can some of you fill me in?

1) How often do they need replaced? (im not running a cat)
You usually replace them only when they fail; they're not generally a regular maintenance item. When they fail, they usually throw some sort of DTC code. It seems, judging by comments on FF, life expectancy varies greatly - 60K miles on up.

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2) I have a cheater on the one o2, but would it still need replacing? (how exactly does the cheater work?)
To understand how a cheater works, you need to understand how an O2 sensor works. The upstream (of the catalyst) sensor measures the proportion of O2 is in the exhaust immediately upon exiting the combustion chamber and, put simply, converts it into a voltage signal. The upstream sensor feedbacks to the ECU and the air/fuel mixture, timing, etc is adjusted accordingly to get the best compromise between power, economy and emissions. Theoretically, after the exhaust gases have been through the cat, the 02 readings between the upstream and downstream sensors should be different. That difference (voltage diff) is read by the ECU. (The second O2 sensor is there primarily to measure the "cleaning" efficiency of the cat.)

If you have no catalyst, the readings between the upstream O2 sensor and downstream O2 sensor should be the same. So "cheaters" fool the ECU by creating a "fake" voltage difference where none exists, usually by introducing a resistance into the sensor circuit somewhere. Other cheaters work by simply moving the O2 sensor out of the exhaust stream with some sort of extension (plug anti-foulers).

An O2 sensor with a cheater attached theoretically should not need replacing; it's actual readings are being altered anyway. Some cheaters simply integrate into the O2 sensor wiring harness and bypass the downstream O2 sensor altogether, I've read.

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3) Can an old/going back o2's effect mpg? (this is why im asking questions)
Not sure what you mean by "going back."
An old or worn upstream O2 sensor can effect fuel economy by making the ECU think the a/f ratio is a value it isn't and can consequently cause the engine to run too lean or too rich as it tries to compensate. A failing downstream sensor shouldn't affect how the car runs but should generate a "failed catalyst" type CEL for emissions purposes.
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Old 01-19-2011, 01:04 AM   #5
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^^opps i meant going bad o2

thank you for your response. +1 rep
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Old 01-19-2011, 09:09 AM   #6
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I am a firm believer in replacing the front sensor about every 25k miles.... and I will NOT use anything but a Ford sensor.

Why you ask??? Because the Ford sensors are calibrated differently than the Bosch generic sensors and they do effect performance (found that out with the Turbo car)

Not only will it effect performance it also effects mileage.

With the cheater there is no reason to replace the rear sensor... its only there to tell the ECU how efficient the cat is anyways.

I know alot of people wont change out the front sensor until it fails, but we have datalogged fairly new sensors that were already getting slow and not responding as they should without throwing a code.
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Old 01-19-2011, 09:49 AM   #7
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Bosch recommends changing the sensors every 100k but as far as I know Ford has no replacement interval on the Focus. I had my cat off last fall and both sensors looked like new (02 zx5) so I don't plan to replace unless I have a cel or drop in mpg.
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Old 01-19-2011, 09:59 AM   #8
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Andrew I need to do my front sensor too. GARAGE DAY!
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Old 01-19-2011, 10:19 AM   #9
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^ben it will take like 2 mins to change it, i have other things we can do on garage day!!! sub/suspension.

i think i'll go get a ford one today.
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Old 01-19-2011, 10:57 AM   #10
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This is again where a xcal3 flasher datalogger would come in handy it can check the o2 sensor and tell you if your running rich/lean at idle and cruise and tell you if the o2 sensor is slow responding and needing changing

Tom
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