Anybody ever heard of this one? I had it scanned at an AutoZone and thats the code that came up, something about fuel trim but there is a list on the Chevrolet forum came up with P1127 Heated Oxygen Sensor Heater not on During KOER Test. Now here is the REAL funky part, the SES light comes on with this code only when I drive on the highway and ONLY on the way home, i.e. the second time I hit the highway in a given day and it comes on at nearly the same spot on the road every time. 3 starts later and it's gone.

 

First read this link: P1127 Exhaust Not Warm Enough, Downstream Sensor Not Tested OBDII Engine Light Trouble Code | Engine-Codes.com

NEVER go by the title or description of a DTC, especially as interpreted by an auto parts store. First thing to do should always type the code into Google, then follow a few of the links.

The conditions for running the various OBDII tests are different and that's why they seem to appear at odd times.

If you are noticing a new loud noise under the hood, I would look at the flex pipe.

 

The more I look the more I think the number was mis-read. Get a second reading from a different person.