My 2011 Sierra, 169K miles old, just started to stumble on cold starts about 2 weeks ago. Today I got the P0175 code. There's plenty of troubleshooting to do for that code and I'll start on it asap (it's cold in MI). This is the first issue I've had with the motor ever and am wondering if one event could be causing this. About 5 weeks ago I inadvertently filled up with 93 octane. That was the first time in 479 fuel fill ups that I've deviated from 87 octane. The motor has always started on the first turn whether with key or remote start since new. But in the past month it takes a couple of turns to start up and occasionally stumbles once before smoothing out. There might be a slight miss at idle, but driveability is still good and no lack of power. The P0175 today was the first SES code to come on. It's got a K&N airfilter since 1k miles and the only other change has been plugs and wires at 160K.