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.
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