I had my car checked up last year and the mechanic said that I will have to replace the oxgyen sensor. I at that time did not have it replaced. Recently I noticed that my car Mini Cooper 2011 started jerking while accelerating and also too many jerks during idling with the AC is on, it looks like that spark plugs have been plugged by the carbon built up or the spark coils are bad.

Assuming that the spark plugs have been clogged leave one conclusion that the O2 sensors are gone bad, now Mini cooper has upstream and downstream sensor, so I would like to know which one is more suceptible to going bad? Replacing both is gonna be a financial nightmare. I had replaced spark plugs just last year and now I will have to replace them again so as far as O2 is concerned I want know which one should I replace as I cant replace both.

  • 1
    You could always try cleaning your spark plugs. Just takes a little bit of carb cleaner and elbow grease. This would save you some money. Aug 27, 2020 at 19:13
  • well im actually noob thats why i get ripped of by these mechanics. I have already purchased the plugs but to ensure they dont go bad again due to incomplete or bad combustion i want to try changing o2 sensor this time.
    – gfdsal
    Aug 27, 2020 at 19:29
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    It might not be the O2 sensors at all. You should try cleaning (or have cleaned) the MAF if it is so equipped, as well as the upper intake tract (like a Seafoam treatment). Gunk on your sensors can cause fueling issues. While the O2 sensors are at the forefront of getting the engine to run correctly, they aren't the only thing. If your car has over 100k miles on it and the O2 sensors have never been changed, this is a maintenance item which should be done anyway. If it's under that, I'd consider other things first. Aug 27, 2020 at 19:46
  • @Pᴀᴜʟsᴛᴇʀ2, you literally saved me from spending more on sensor, so I will give priority on having MAF cleaned, maybe fuel injectors as well as i believe they might be clogged as well as I heard someone say that injectors also get jammed.
    – gfdsal
    Aug 27, 2020 at 20:26

1 Answer 1


The 02 sensors can be tested multiple ways. A mechanic will primarily rely on his scanner for diagnosis. You only need to change the one that is not working properly. If both sensors are confirmed defective and you can only afford one, then the upstream should be changed first.

  • Allright, any explanation why upstream first? I thought downstream O2 sensors usually go bad due to soot from the exhaust..
    – gfdsal
    Aug 27, 2020 at 19:30
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    @gfdsal - Because in most cars, the upstream sensors do all the heavy lifting (affect the fuel flow). The downstream sensors are only checking the function of the catalytic converter. Some newer cars use the downstream sensors for fuel calibration, but I don't believe your Cooper is one of them. Aug 27, 2020 at 19:42
  • @Pᴀᴜʟsᴛᴇʀ2, that makes lots of sense!
    – gfdsal
    Aug 27, 2020 at 20:27

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