The speed sensor is located in the conductor plate. When the tcm experiences erratic or no signals from the speed sensor this can cause limp mode. I would assume the mechanic checked the external wiring to the tcm. And checked the tcm location for water. If there is no water in the floorboard and the wires are solid then it might be a faulty tcm. However on the 722.6 it is most likely that the conductor plate needs to be replaced. Along with the plastic piece the seals the wiring going into the trans. I had that exact same thing happen on my 04 at 158,000 miles. Any modern scanner can find codes in the tcm, ecu and other modules on the crossfire.