I replaced 02 sensors at 60,000 miles on my x5 because of mods I have done. I will replace the sensors on my wife's 335 at 100,000 miles if we keep it that long. When I have changed them on other BMWs we've had in the past at around 100,000 miles the butt dyno said performance improved but I was never sure. Seemed like mileage was a little better but based on mix of city/highway driving I can't say for sure either.

My response was based on your concern about the cost. Just suggesting you still have the option to save the money since there is no indication they are failing.

Obvious, there can be performance deterioration of 02 sensors without a code or check engine light. And yes, it is recommended to replace sensors after 100,000 miles. That doesn't mean at 100,000 miles they drop dead. Might consider checking them to see if the voltage is per spec. You could also have an emission test done to see if engine is in compliance. Sometimes the exhaust pipes are sooty indicating a rich condition if engine isn't burning oil.

It is reasonable to assume but not expect you will get some improvement if only mileage. If you replace the sensors I would consider replacing spark plugs, fuel and air filters and at least cleaning the MAF if they haven't been addressed .

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas