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I have a 2005 Chevy cobalt LT that has a problem. I've replaced the front o2 sensor three times now in the course of 3 months. The cel keeps coming on saying its giving a bad reading. The catalytic convertor is brand new on it. The dealership replaced it about 7 months ago. What else could've making the o2 sensor go out? Once I replace it, its fine for about a month. Also, cold or wet weather occassionally turns the cel off.
 

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Ok..........

Well since I asked that I will explain why. If you are running low octane gas, you are more than likely fouling your spark plugs causing you to run a bit too rich. When you run too rich, you foul the O2 sensors as well. However, if you run better than midgrade gasoline, you have stumped me. The only advice from me to you would to upgrade your O2 sensors if that is the case.

Solution: Check spark plugs and if fouled, replace immediately. And also change gas to premium upon next fill up and continue using premium grade. Run her a good bit redlining a couple times to release the excess CO2 that may be left in the catalytic converter.
 

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Premium gas in a 4 cylinder car? It says right in the book to not use premium gas. Since the last post, the problem has been intermittent, weather dependant.

The light will be off for 3 or 4 days, than suddenly one day out of nowhere turn on again. It depends on the weather the condition of the light or not. The spark plugs are fine, tested all of them. My wife told me one time she stopped suddenly for a red light and her 'BRAKE' light came on. Sounds to me like a short somewhere or something. Is there a common short/ground issue somewhere in these cars?
 

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For your brake light issue, the fluid might be low. If not, have a shop diagnose the condition of the brakes.

And I only asked what type of gas is because it varies for the trim of the vehicle as well. If you have a SC (supercharger), of course you want premium gas. It is a forced induced vehicle that requires better combustion and a higher octane fuel will suit that with less residue on the spark plugs. But since you are an LT, it seems like it doesn't require to have premium. But it would be better to test to see if makes a difference. At only a couple cents more a gallon, it is a pretty cheap way to test. And since you have diagnosed the spark plugs, what brand are they? AC Delco is the original installed plug, ok for the vehicle but I have found out that E3 spark plugs burn the fuel more efficiently and result in better economy and less residue and longer life.
 

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u r making it sound like putting 93 in a 4 banger is a bad idea lol all us guys who drive SS's must use 93 lol but i would go back to the dealer tell them exactily whats going on that its been replaced that many times because i believe it gets to a point when they keep replacing the same part and it goes bad it will usually fallow in your favor they did that with my other cobalt the MAF sensor kept getting screwed up and they actually gave my sister a new cobalt LT
Premium gas in a 4 cylinder car? It says right in the book to not use premium gas. Since the last post, the problem has been intermittent, weather dependant.

The light will be off for 3 or 4 days, than suddenly one day out of nowhere turn on again. It depends on the weather the condition of the light or not. The spark plugs are fine, tested all of them. My wife told me one time she stopped suddenly for a red light and her 'BRAKE' light came on. Sounds to me like a short somewhere or something. Is there a common short/ground issue somewhere in these cars?
 

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What brand of O2 sensor are you buying? BOSCH? If so, ask to see if you can get an NTK (NGK's brand). Also, make sure your connection on the plug-in does not have a bent terminal. (it happens)
 
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