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Oxygen Sensor Replacement: Upstream & Downstream of Cats

Discussion in 'E90/E92/E93 M3 (2008-2013)' started by 106598, Mar 9, 2019.

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    Hello All!
    Need to change out my O2 sensors on my high mileage 2008 E92 M3 S65 V-8. Approaching 130,000 and didn't make Cal smog yesterday. All good except: Evaporative System, Secondary Air & Oxygen Sensor. Ordered new gas cap with blue o-ring.
    Question: How do I access the upstream O2 sensors? From under the car or from above? I raised the car up yesterday with my floor jack. I saw the (2) downstream O2 sensors and they are relatively within reach. The upstream ones are the ones I'm concerned about. The left bank/Driver's side sensor is in question as I had gotten "2B49/not present" Misfire impairing exhaust emission cyl 8" message on my scanner before smog yesterday. Engine has a skip and loss of power but no warning lights on dash. Changed out camshaft position sensors(4) on both banks & spark plugs(22,000 miles on current plugs). Cranked it up and engine ran great again. No skipping and plenty of power. Cleared codes with scanner and then the smog inspection issues yesterday.
    Charlson89 your thoughts? Bad O2 sensor on left bank?
    Thank You!
    BMWCCA Member #106598
    VIN: PY39339
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    Not something I've done since I don't have your car, but between the following three, my guess would be from underneath - the realoem parts diagrams quite often offer a bit more clarity than this particular one, it appears the pre-cats sensors are pretty much immediately pre-cat; part #1 in the diagram -


    The pics on this ebay auction look to be helpful - https://www.ebay.com/i/273694554295?chn=ps

    Will add pic below for future reference -

    From this youtube vid where he's getting to the rear 02 sensors, looks like perhaps that aluminum cover plate might need to come off to access the front 02 sensors; take a look from the topside though - if you can lay eyes on them, maybe there's a chance they could be got at from the top; my guess though would be not enough clearance.



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    Thank You, MGarrison for the very helpful information! Much appreciate it!
    John M. Coates, Jr.
    BMWCCA Member #106598
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    Yes from underneath is the best option and use a short o2 sensor socket if always best since they will probably come out hard. As for not passing emissions what was the reason for not passing the monitors were set yet or did they have hard faults?

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