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Hey Guys, looking for advice/any similar experiences you have

A week back, I took my 2008 135i to the dealership as it was having a low tone sounding coming when the car was on low rpm. They identified the issue to be a result of turbo and under the extended warranty replaced the turbos. So far so good. I didn't have to pay.

When taking delivery of the car, I noticed that the 'service engine soon' light was on. Service consultant reset it and told me it should be fine. The light came back when I restarted the car after reaching home.

Back to dealership after a couple of days an they tell me they have to replace an O2 sensor (Bank 1 post Catalytic converter oxygen sensor) and wants me to pay $600 for it. My thought was - the car never threw an error before..shouldn't they be fixing it for me since it happened while the car was with them? Their version is its just a coincidence!!

So there question I have is, during the turbo replacement, which I believe is a big job, are there chances of O2 sensors impacted? In other words, am I right to suspect this definitely is an undesired outcome ( to err is humane, so is it to own it up, I say) Should I be paying for a situation like this?

Any advice?