P0776 Error Code in 2011 Toyota RAV4: Transmission Leak and Post-Fluid Change Issues

My son recently purchased a used 2011 AWD 4.0L Toyota RAV4 with approximately 140,000 miles. The vehicle was sold at a great price, but over the past six months it has developed several serious issues. Initially, it performed well—no shifting or acceleration problems were noticed—despite frequent use on rough gravel roads and regular hauling of heavy loads. Recently, a transmission fluid leak appeared. When checked, the fluid was grey-black with no noticeable burnt odor or visible metal particles. After topping off the level, we stopped driving the vehicle entirely. Our local auto shop recommended visiting AAMCO for transmission work. At the Toyota dealership, they identified burnt transmission fluid and axle seal leaks, providing an estimate of $1,400 to flush the transmission and replace axle seals—adding that further repairs might be needed. We then went to AAMCO, where they diagnosed a leak in both the transmission oil pan and driver-side axle seal. They replaced the transmission fluid, pan gasket, filter, and axle seal for $554. I believe a full transmission flush was not performed—while the fluid appearance improved, it does not look clean or pink like new fluid should. After picking up the vehicle, my son noticed several issues: a noticeable downshift at 25 mph with increased RPMs, followed by activation of the check engine light, 4WD, and traction control lights. Reverse gear now fails to engage entirely—though forward drive still works. Using an OBD2 device, we confirmed only the P0776 error code. There is also a persistent transmission fluid leak, with a visible 6-inch spot on asphalt within 48+ hours of parking—a level comparable to before repairs. We initially suspected AAMCO made a mistake. However, research suggests that changing old, burnt transmission fluid—especially without a proper flush—can exacerbate existing internal damage due to the reduced viscosity of new fluid. This may explain why the transmission is now failing to engage properly. So while I don’t believe AAMCO caused internal damage, they clearly did not resolve the leak. Should they be warning customers that simple fluid changes could reveal or worsen transmission problems? Are AAMCO shops consistently reliable—or are there good and bad ones in a given area? Could the P0776 code stem from a faulty solenoid, damaged wiring, or an internal transmission issue like a clogged filter or valve body? Could it be due to a defective PCM (Engine Control Module)? Given that no transmission symptoms existed prior to the leak, how could an ECM failure suddenly appear after fluid service? Can AAMCO access and override the transmission control module to test whether the problem lies with the solenoid, ECM, or internal transmission components? Are P0776 issues common in 2011 RAV4s—or was this primarily a concern in earlier model years (e.g., 2001–2003), which were later addressed? We are committed to getting the vehicle properly repaired and will pursue any necessary steps—without accepting dealer pricing.