Automatic transmission shifting problem, code P0715

I have the vehicle listed above with 1KR engine and a 5 speed automatic transmission. Recently, the car just stopped shifting into second, third and fourth gears after a sudden jerk on the transmission as I was driving. There were check engine light and O/D lights on the dashboard and a P0715 DTC was retrieved. My mechanic tried replacing the input/turbine speed sensor but there was no success. He concluded the gearbox was 'dead'. I have replaced the whole gearbox unit but the problem persists. The mechanic claims the new transmission is still faulty.
Kindly advise me on what to do.