Hello I'm Neal and I am here to help resolve your issue. If I cannot help I will opt out and refer your question to another expert.
That is a known issue and this condition is caused by a broken clutch wave plate allowing the piston to over-stroke, causing it not to apply pressure. This normally will not occur before approximately 20,000 mi. This can also lead to the speed sensors not working and can damage other transmission parts..
Repair requires the transmission be rebuilt unfortunately. I can provide the entire tsb if you like.
Here you go.
TECHNICALBulletin No.: 09-07-30-012BDate: March 08, 2011Subject: No Reverse, 3rd or 5th, Check Engine Light Illuminated, DTC P0776 (Replace 3-5 Reverse Clutch Plate (Waved))Models:2008-2009 Buick Enclave 2008-2009 Chevrolet Equinox, Malibu2009 Chevrolet Traverse2007-2009 GMC Acadia2007-2009 Pontiac G62008-2009 Pontiac Torrent2007-2009 Saturn AURA, OUTLOOK2008-2009 Saturn VUEAll Equipped with 6T70/75 6-Speed Automatic Transmission (RPOs MH2, MH4, MH6 or MY9)Supercede:This bulletin is being revised to update the information. Please discard Corporate Bulletin Number 09-07-30-012A (Section 07 - Transmission/Transaxle). ConditionSome customers may comment about an SES light and/or no reverse and may also comment on a slip / flare or harsh shifts in drive range 3rd and/or 5th gear. Upon investigation, the technician may find DTC P0776 (Clutch Pressure Control (PC) Solenoid 2 - Stuck Off) set. Any one of the following codes may set independent of each other as a result of this wave plate breaking: P0777, P2723, P0717, P0716, P2715, P2714.CauseThis condition may be caused by a broken 35R clutch wave plate allowing the apply piston to over-stroke, causing the piston to leak and causing loss of apply. This normally will not occur before approximately 32,000 km (20,000 mi). Debris generated by the condition can affect the operation of the speed sensors and other clutches.CorrectionImportantIt is recommended that a new plate be installed any time the transmission is disassembled for any reason.
A new 3-5 reverse clutch plate (WAVED), P/N 24254103 (423), has been released to address this condition. Refer to SI for the appropriate repair instructions.A broken 35R wave plate can generate varying amounts of debris inside the transmission. In most cases this debris has not passed through the pump fluid filter (201) so has not caused damage to the pump, valve bodies or torque converter. This debris can be cleaned and the transmission successfully repaired.In summary the following steps should be taken: 1. After removing the transmission from the vehicle, remove the case cover assembly (35). Inspect the case cover for damage. This part is serviced and can be replaced if it is damaged. 2. Remove the 35R / 456 clutch housing assembly (422) and inspect it for a broken 35R wave plate (423). 3. Inspect the case (21) for damage. If the case is damaged, the transmission must be replaced as the case is not serviced. If the case is not damaged, proceed with the tear down and inspection. Any possible damage is typically confined to the case lugs. If the clutch plates can move freely to engage or release the clutch packs, the transmission should not be replaced. If the snap ring grooves are not damaged and will securely retain the snap rings, the transmission should not be replaced. Peening or wear of the edge of the case lugs that engage the clutch plates is normal and acceptable and not cause for transmission replacement. 4. Remove the control valve body cover (1), control solenoid (w/body and TCM) valve assembly (15) and filter plate assembly (16). 5. Inspect the filter plate assembly (16) for the presence of metallic debris on the screens. - If there is no debris on the screens, the pump fluid filter was effective and there is no debris in the pump or torque converter. Therefore the pump, torque converter and valve body do not need to be replaced. The transmission should be repaired. - If there is debris on the screens, the pump and torque converter should be inspected further. Refer to SI procedure Transmission Fluid Pump Cleaning and Inspection for the pump. Refer to SI procedure Torque converter Diagnosis for the section Torque Converter Replacement Guide. - Fine metal particles that you cannot feel (traces of metal flakes/gray color to the fluid) are not reasons to replace the torque converter. - Metal chips/debris that you can feel or that are found in the converter, or when flushing the cooler and cooler lines, indicate the torque converter should be replaced. 6. The valve body must be disassembled and cleaned, refer to the latest version of Corporate Bulletin Number 08-07-30-005 for direction. 7. The transmission case halves must be separated, internal components removed and all parts thoroughly cleaned. Be sure to clean the speed sensors, both input (ISS) and output (OSS). These sensors are magnetic and will attach and hold any ferrous material. Failure to clean these sensors may result in DTCs. A new pump fluid filter must be installed.