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Old 07-18-2007, 08:53 PM
 
Join Date: Jul 2007
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Default o2 sensor fault but no DTC code or MIL light

Ijust bought a 1997 Volvo 850 with approximately 95,000 miles. When I put my Elmscan OBD-2 reader on it, the software indicates that the car is in "closed loop o2 sensor fault" status. However, the reader indicates that there are no engine fault codes to retrieve. The check engine (MIL) light is off.

I notice that I am getting good readings from the O2 sensor number one. But there does not appear to be a reading from a second O2 sensor.

I am guessing the downstream O2 sensor must have come unplugged or gone bad. (The previous owner just had the muffler redone -- it is possible the sensor was unplugged and never replugged in). But I am puzzled about whythe check engine soon light does not turn on if one of the O2 sensors is bad. This makes me wonder if the problem is different than what I think. (The engine is running great, and other than what the scanner tells me, I have no indication that there even is a problem.)

Does anyone know if this is normal? And do these symptoms sound like a bad o2 sensor?
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Old 07-19-2007, 02:36 AM
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JPN JPN is offline
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Default RE: o2 sensor fault but no DTC code or MIL light

Welcome to the forum.

>I just bought a 1997 Volvo 850 with approximately 95,000 miles.
Sounds like a good deal. Make sure the timing belt has been replaced at 70,000 miles or in the last 5 years. If there is no definite record, I would replace it anyway.

I personally am skeptical about 850's self-diagnostic sometimes, as it turns on CEL for trivial reasons.

BTW, is your 850 turbo? Turboed 850 has two O2 sensors (front & rear), whereas NA 850 has only one (may be different on '97 models, but on '94 models that's the set-up).

If the car runs fine and CEL is off, I wouldn't worry about it (unless the light bulb for CEL is shot; does it come on when you turn the key to position II?).

Or if it deprives you of sleep, you can visit a local AutoZone and have them do a free diagnostic with their scanner (the one by O'Hare uses Actron CP9175, which is a decent OBD-II scanner). Or, donate some $ to a dealer.

I hope this helps.


JPN
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Old 07-19-2007, 02:36 AM
 
 
 
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1997, 850, 97, closed, code, codes, dtc, fault, loop, o2, oxygen, reading, sensor, unpluged, volvo


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