why are the 2012 G37 transmissions made before August not compatible with after?

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your vehicle. In 2012, Infiniti made changes to the G37's 7-speed automatic transmission that affected compatibility. The difference between the two transmission codes (31020-X438C and 31020-X457B) typically reflects internal updates like electronics, software, or mechanical adjustments. These changes, often implemented mid-year, are designed to improve performance, durability, or compatibility with updated engine and control modules. If you attempt to use an incompatible transmission, you might experience issues with shifting, communication between the transmission control module and the engine, or even drivetrain malfunctions. It's crucial to match the exact transmission model for reliable operation and to prevent potential warranty or drivability issues. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!