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I have an '07 Chevy Cobalt that I've had since 2013 and it's been a great car until recently. About a year ago I had a code for p2a01 h02s performance sensor. I was going to replace it but the check engine light went off and only popped up every once in a while so I didn't think it was a big issue. A couple weeks ago my check engine light came back on and I figured it was just the o2 sensor again but when I took it in to my mechanic to get my rear shocks replaced he ran the check engine light and I got three codes this time. P0300 engine misfire, p1174 fuel trim cylinder balance, and p2a01 h02s performance sensor. I had misfires on cylinder 2 and 3 and replaced all 4 spark plugs. That fixed the misfire on 3 but I'm still getting a misfire on 2. It's not the coil packs bc we tested it by switching them to the other cylinders and they were fine. Did a compression test and had very low compression on cylinder 2. Not exactly sure where to go from here because I'm hesitant to go back to my mechanic after having spent a pretty good amount of money already and it's still not fixed. He recommended I get a fuel injector flush but then later said he didn't think it would fix the issue. He wants to remove my engine block and see if he can find the exact cause but the labor alone in that is more than I'm willing to pay. As soon as I first got the news about the engine misfire I got some BG's 44k fuel treatment. Seems to be helping a little because my engine light has gone off a few times but eventually always comes back. When it's off the engine doesn't seem to misfire as often. When it comes back on, obviously it's worse. It seems to be ok when I first start driving but if I idle for too long if I'm parked or stuck at a traffic light I'm getting a lot of rough shaking. Looking through other forums Ive thought about cleaning my throttle body and seeing if that helps. I'm also wondering if I should go ahead and fix the o2 sensors? I don't want to replace them if I can fix the engine misfire first and possibly that might fix the o2 codes? I've gotten so many different opinions and advice from different mechanics I know and everyone seems to be telling me something completely different. Anybody out there who has dealt with this same issue and had it resolved? I'm getting sick and tired of spending money only to have nothing fixed.
 

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Low compression on cylinder 2 is not going to fix itself. If a valve isn't seating right the only way is to lift the head and machine the head or replace it completely with a remanufactured or new.
 
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