I have a 2009 Pontiac Vibe. The Check Engine light was on and I took it in to a mechanic. He told me that the downstream O2 sensor was not responding and that he physically removed it. He said that he didn't replace it, since he didn't have the right part, but to not worry since the car will drive just fine. This struck me as a huge red flag. The check engine light, strangely enough, was off when I started the car up and it certainly does drive OK. I've since driven about 100km over a 24 hour period and no light.
Here's my confusion (beyond the sketchy mechanic): if the sensor wasn't responding when it was in the car and the check engine light turned on, why isn't the light on now that it's out of the car?
I suspect something like this happened: he got one of his subordinates to do the job, and they realized that the sensor wasn't working. They put it back in but communication got crossed and the main mechanic thought that they didn't or something. Possibly the sensor was OK maybe loose, so putting it back it fixed the problem. Is that even possible?