Sudden increase in gas mileage without A/C

I have a 2006 Pontiac Vibe. About 2 months ago, someone stole its oxygen sensor (there was a mass catalytic converter theft in that parking lot but they accidentally took the oxygen sensor from my car). We got the oxygen sensor replaced. Ever since that, a strange thing has been going on. When I turn on the fan (the A/C is off)and the temperature setting is at the coolest, the car cools down as though the A/C is on. This happens ONLY when the fan vent is towards the front wind shield (defrost?)It does not happen in any other setting. Do you think that the A/C is being turned on (without me actually switching the A/C on) when the air vent is towards the windshield? One thing to note is that the A/C does work properly when I actually turn it on, regardless of the position of the vent. Thanks.

When you set your HVAC system to the “defrost” setting, the A/C compressor is activated, in order to help defog the windshield. This has undoubtedly always happened when you turned on the defroster, even if you didn’t previously notice it. If you don’t believe me, just open your Owner’s Manual to the section for the HVAC system, and it should confirm this information.

I am a bit confused about the title of the post vs. the content of it. In the body of your post, you don’t make reference to anything regarding gas mileage. Do you also have a question regarding gas mileage?

I am sorry about the discrepancy. I actually saw a slight decrease in the gas mileage also and I was not sure if this was because of the above ‘A/C’ problem. Since the problem is not that big, I removed that from my query but forgot to change the title.
Thanks for your answer.