My fiance has an 07 Sonata v6, yesterday she called me to say that the check engine light was flashing and the car was running rough. I went to her work, put some dry gas in it, and took it for a spin, engine light was on, but not flashing, car was running fine. I went to Advanced Auto Parts and had them check the code, they said it came back as a bad O2 sensor, unsure which, and car was mis-firing on cylinders 2 and 4. The guy couldn't tell me which sensor it was, but said it was probably one of the ones in one of the manidfolds.
Any ideas which sensor it is?

when she was coming home the light was flashing and car running rough again, she let is sit for 20 minutes, and when started again light was off, and car was running like new...could the dry gas have cleaned any gunk off the malfunctioning sensor?

 

Maybe, or perhaps some water got in to the fuel line from condensation or bad gas. O2 sensor problems usually generate several error codes, indicating which cylinder bank and sensor in that bank is having problems. Either the fellow at Advanced Auto had a cheap code reader that couldn't decypher the codes or he didn't know what he was talking about. If the codes were cleared out and the car appears to be running normal, I would drive it until the problem re-appears and then have someone else read the codes. If unrelated to one of the O2 sensors, I am guessing the problem may have been resolved with the dry gas. A lot of good O2 sensors get replaced for no real good reason.

 

Dry gas won't clean off a O2 sensor, it will pick up and suspend water and some minor contaminants from a crappy fuel supply. If you routinely purchase fuel from a 'cut-rate' station you may have gotten a bad load of gas. It happens due to pipe lines being used for more fluids than just gas. Hopefully the gas cap was on fully thru all this.

Next fill up, I'd add a full can, (12 oz), of Sea Foam to the tank, (approx. $7.50), take the car home and disconnect the battery for about 20 minutes to clear any codes, and then see what happens as you then drive the car normally.