EGR system P1400 code?

Your Tahoe (5.3L L83 EcoTec3) does NOT use a conventional EGR valve like older GM trucks. Instead, GM uses Internal EGR, which is handled inside the engine through camshaft timing (VVT). What causes this problem is a dirty or sticking VVT oil control valve/solenoid, or the engine il is low or needs to be changed. Here is the location of the valve so you can remove it and check the oil control screen for metal debris. If it is clean I would replace the VVT valve. You also can use a scanner to watch the commands and response to see if it is working as well. Check out the images (below). Please upload an image or a short video in your response, so I can see what's going on.