I have a BMW E46 M3 with 100k miles that's smelling rich when cold and seems to be getting worse mileage than it used to. It has passed the UK MOT emissions check recently, the plugs seem fine and I've had no codes.

I was thinking that it could be the O2 sensors, as far as I know they haven't been replaced before, and my understanding is that over time they degrade and give less accurate data to the ECU. My mechanics, who know the M3 pretty well and have generally been on the money before, have told me that O2 sensors just break, i.e. they work until they don't and until they break they don't need to be replaced. This is at odds with my understanding.

Do 'relatively' modern O2 sensors slowly go bad, or do they just stop working?

If they do slowly go bad how would I determine if this is the case on my car? Replacing them is a pain I'd rather avoid, would analysis of OBD data be useful?

2 Answers 2


O2 sensors become lazy over time. They do not respond as quickly as they do when they are new. If you have a graph plot of them when new, you'd most likely be able to tell the difference, however, just looking at a graph of old O2 units probably wouldn't give you a clue because you'd not have anything to compare it to. Being lazy, the O2 units do not provide the best data to the computer, which means your fuel mileage will suffer. Sooner or later they will fail, though this will happen much later in their lifespan after they are really of no use to the computer.

O2 sensors should be considered a maintenance item, just like you'd change your oil. While they may be a PITB to change, changing them will ensure your vehicle can run the best it can. Considering most vehicle manufacturers lay a replacement schedule around 100k miles, changing them shouldn't happen that often, so the pain which is changing them happens, then doesn't happen for a period of time after, lessening the pain (hopefully). The bigger pain here, I think, is if you are smelling rich, this means you're dumping excess fuel onto your catalytic converters, which they don't like. This will cause them to die prematurely. Considering what a PITB they are to change both physically and in the pocketbook, changing out the O2's should be a whole lot less painful. However, this is your choice. Not changing the O2's will not destroy your vehicle. It may cause bigger issues down the line for you, but that's the choice we all have to make when we put off routine maintenance.

  • I have this problem right now. I’d replace the bank 2 sensor if I could get the old one to unstick!
    – Bob Cross
    Aug 15, 2020 at 17:45

It may be anecdotal or marketing hype. I did see a demonstration where they compared sensors with 80,000 miles on them with their Brand X new units. While the final results were the same the new units reacted faster than the originals.

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