No First And Reverse Gears

WARNING: This page is about a different car, the 2018 GMC Yukon XL, 2018 GMC Yukon, 2018 Chevrolet Tahoe, 2018 Chevrolet Suburban 3500 HD, and 2018 Chevrolet Suburban. 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).
Pump Cover (219)	Inspect for sticking or damaged TCC control valve (227).