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slick_willy
Dodge Dakota
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10/09/2005
14:52:38
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 Subject: o2 bad but no 'check engine' ?
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'99 Dak sport with 111k on the clock. 3.9L manual 2WD 22 gal. gas tank w/typical distance traveled between fillup's ~400 miles ...
Now, only getting about 325 miles per tankful. Mostly highway driving around 60 MPH. Checked tires, PCV, and air filter ... all were good. Never had a sensor go bad, but I'm leaning toward it being an O2 sensor as I can't explain why the drop in mileage. No check engine light came on, and Autozone scanner provides no codes. Truck doesn't run any differently than it always has. No shuttering, backfiring or anything like that. Last maintenace was upper ball joint about 2k miles ago and replaced brake rotor's about 1k miles ago. Truck is a daily driver.
My question is, is it possible for an O2 sensor to fail, and not cause the check engine light to come on ? Would a dealer scan provide different results ?
In checking the archives, there are lots of threads about O2 sensor's, but not much info with correlation between check engine light and O2 sensor going bad. If the sensor is bad, I would assume it's the sensor before the cat, right ? Am I wrong in my troubleshooting, or have I forgot anything ?
Thanks and regards
Slick_Willy
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Adam
Dodge Dakota
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10/09/2005
16:03:42
| RE: o2 bad but no 'check engine' ?
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That's weird because we are in almost the same exact position. Went to the dealer and they gave me a paper saying I had a 22 gal tank but they're wrong, on empty the most I put in was 17 gal...anyways, 112000 on my '00 3.9 4wd.
I dropped to about 15.5 mpg from 17.5-18 and when I changed my oil I could smell gas. I posted here somewhere about it and was told it could be a bad O2 sensor(pre cat) and it was making me run too rich. We'll find out when I get paid and I get a new one. None of my lights have come either so I'm hoping this is the case.
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slick_willy
Dodge Dakota
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10/09/2005
19:24:35
| RE: o2 bad but no 'check engine' ?
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I know I have a 22 gallon gas tank. My typical fill-up is 18-19 gallons. I've recently changed my oil, and I had no gas smell.
Let me know what you find when you change O2 sensor. I'm hoping others will chime in with their experience concrning sensor's also.
Slick_Willy
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slick_willy
Dodge Dakota
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10/10/2005
14:56:56
|  RE: o2 bad but no 'check engine' ?
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Can anybody else assist with more information ? My hope is that I can solve this problem without having to swap too many parts.
Thanks and regards
Slick_Willy
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Yep
Dodge Dakota
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10/10/2005
15:15:49
| RE: o2 bad but no 'check engine' ?
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O2 sensor can be going bad without it showing up as a code. Happened to me and after changing, everything went back to normal. Computers are only as smart as the people who programmed them and also if all chips are working properly.
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daddio
Dodge Dakota
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10/10/2005
15:29:28
| RE: o2 bad but no 'check engine' ?
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O2 sensors can get "lazy", not function optimally but still be within acceptable parameters for the PCM to think it's still OK. it's happened to me before just not on my dak. no trouble code but checked with VOM according to service manual and it was bad.
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slick_willy
Dodge Dakota
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10/10/2005
17:02:38
|  RE: o2 bad but no 'check engine' ?
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Thanks for the input. I will check to see if the sensor is available at autozone. If not, I guess's it's to the dealer :)
Regards
Slick_willy
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Adam
Dodge Dakota
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10/10/2005
18:52:38
| RE: o2 bad but no 'check engine' ?
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Just got my O2 sensor today...just waiting for truck to cool down b4 I go play with it. Autozone has them for $81 OEM style and $68 if u want to splice ur own wires. I'm cheap so I decided to splice my own. Hope this works.
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?
Dodge Dakota
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10/11/2005
10:55:49
| RE: o2 bad but no 'check engine' ?
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http://www.dodgeram.org/tech/gas/Trouble/O2_sensor.htm
see above for full testing info , but in general
YES - sensor can still sort of work but poorly
and leads to poor fuel mileage. Anything over about 60,000 miles = suspect..
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Slick_Willy
Dodge Dakota
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11/02/2005
22:08:35
| RE: o2 bad but no 'check engine' ?
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I just wanted to follow-up with the forum and let you know that I fixed the problem with replacing the pre-cat O2 sensor. After 2 weeks of driving, my typical gas mileage of ~400 miles/tank is back.
Thanks again,
Regards
Slick_willy
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