Just picked up my G37 Sedan last weekend.

Issue 1: Today, I drive to the store and notice there is very little power. I put the gas to the floor, no change. It drives and shifts normally, but with very little power. It maxes out around 45mph, and shifts around 2,500rpm.
Issue 2: The rear LED lights stay on and will not turn off.
Issue 3: Pressing the START button without my foot on the brake will start the car - it will not go into ACC mode.

This all happened this morning right after having my windshield replaced (per my purchase agreement from the dealership), and I have no indicator as to why. It feels like it suddenly went into a Limp Mode. Any suggestions will help!

First thing,.. take it back to the dealer. Limp mode could be for so many different electrical problems and can bus communication issues. If you can not get it to the dealer then get some way to read any codes. Sometimes there might be codes that are not ecu related and they would not show up in the mil,specially TCM and ABS. If your windshield have some electric windshield heat connection and they messed it up during installation then it might be the cause, or they might have pinched some other cable in the process. No way to really know until you read codes or find out what they did. Best luck to you.

Thank you for the response! I was able to limp it to a local dealer (original dealer is 3 1/2 hours away), and the determined it was due to a bad battery. There was a 600 CCA battery that was testing right on the border of bad/good, and come to find out, the car requires a 700 CCA. Swapped the battery, all problems fixed. Dealer said they had never seen anything like it before.