Sorry to keep going on - but READ THE MANUAL!! It talks about the throttle position sensor and how codes can be read from the TCM, and that they're stored in the ECM etc. Have you got any lights on your dash??
If you dont understand it, i would print the manual, or take it on a laptop to the garage or an auto electrician. It's the Nissan service manual that they work through to diagnose problems with the AT transmission!!!!
It could be a faulty sensor, or solenoid, or TCM module or anything. If your car didnt start, would you replace the engine??
You cant have burnt out 2 gear boxes, for them to have the exact same symptoms can you? Burning out a 'box doesnt happen overnight...!