No First and Reverse Gears

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2011 GMC Yukon XL, 2011 GMC Yukon, 2011 Chevrolet Tahoe, 2011 Chevrolet Suburban, and 2011 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

Checks	Causes
A/Trans Fluid Pan Assembly (29)	Verify proper fluid level and condition, inspect transmission assembly for leaks.
Control Valve Upper Body Assembly (304)	Inspect for sticking, damaged, or worn 1st/rev and 4-5-6 clutch regulator valve (327) and spring (326).
Control Valve Lower Body Assembly (307)	Inspect for sticking, damaged, or worn clutch select valve 3 (352) and spring (353).
Control Valve Lower Body Assembly (307)	Inspect for sticking, damaged, or worn clutch select valve 2 (352) and spring (353).
Control Valve Upper Body Assembly (304)	Inspect for sticking, damaged, or worn #2 ball check valve
Center (with 2-6 Low and Reverse Clutch) Support Assembly (67)	Inspect for damaged low and reverse clutch piston assembly (470), spring (469), spring retaining ring (466), and center support assembly (459). Inspect for damaged or worn low and reverse clutch plates (463-465). Inspect for damage low and reverse clutch backing plate retaining ring (461). Inspect for damaged low clutch sprag assembly (467). Holding the sprag assembly with the retaining ring facing up, and the inner race of the sprag assembly fixed, the outer race should freewheel counterclockwise. Inspect for cut, worn, or damaged low clutch sprag seal (468).
Center Support Fluid Passage Seal Assembly (20)	Inspect for worn, cut, or damaged seals. Inspect for proper seal (20) size (6L45/50E MYA/MYB) vs (6L80/90E MYC/MYD).