Originally Posted by
ZeroBoostBuick
If you don't have a vacuum leak it could be a bad upstream 02.
I've seen this when the upstream O2 was bad. Kept telling the ECM the mixture was rich when in fact it was perfect. So the ECM kept pulling fuel, resulting in a lean mix.
Another time I've seen this, the gasoline had too much Ethanol and the car didn't have a Flexfuel sensor. Anything higher than 15% Ethanol will do this. Sometimes even 15% could do it in very cold weather. The cts 2.0 LGT recommends Premium 91+ octane which should have next to no ethanol. But you can safely run Regular gas as long as it doesn't have more than 10% ethanol.